docs: Custom Processors cleanup and expansion (#16838)

* docs: Custom Processors cleanup and expansion

* remove TODO

* remove extra white space

* remove TODO comment

* implement NZ feedback

* Add multiple custom processors

* implement NZ's suggestion about defining `myCustomProcessor`

* add message type

* Apply suggestions from code review

* Update docs/src/extend/custom-processors.md

* Update docs/src/extend/custom-processors.md

Co-authored-by: Brandon Mills <btmills@users.noreply.github.com>

---------

Co-authored-by: Brandon Mills <btmills@users.noreply.github.com>

docs/src/extend/custom-processors.md

@@ -7,11 +7,12 @@ eleventyNavigation:
     order: 2
 
 ---
-You can also create custom processors that tell ESLint how to process files other than JavaScript.
+
+You can also create custom processors that tell ESLint how to process files other than standard JavaScript. For example, you could write a custom processor to extract and process JavaScript from Markdown files ([eslint-plugin-markdown](https://www.npmjs.com/package/eslint-plugin-markdown) includes a custom processor for this).
 
 ## Custom Processor Specification
 
-In order to create a processor, the object that is exported from your module has to conform to the following interface:
+In order to create a custom processor, the object exported from your module has to conform to the following interface:
 
 ```js
 module.exports = {
@@ -46,29 +47,66 @@ module.exports = {
 
 **The `preprocess` method** takes the file contents and filename as arguments, and returns an array of code blocks to lint. The code blocks will be linted separately but still be registered to the filename.
 
-A code block has two properties `text` and `filename`; the `text` property is the content of the block and the `filename` property is the name of the block. Name of the block can be anything, but should include the file extension, that would tell the linter how to process the current block. The linter will check [`--ext` CLI option](../use/command-line-interface#--ext) to see if the current block should be linted, and resolve `overrides` configs to check how to process the current block.
+A code block has two properties `text` and `filename`. The `text` property is the content of the block and the `filename` property is the name of the block. The name of the block can be anything, but should include the file extension, which tells the linter how to process the current block. The linter checks the [`--ext` CLI option](../use/command-line-interface#--ext) to see if the current block should be linted and resolves `overrides` configs to check how to process the current block.
 
-It's up to the plugin to decide if it needs to return just one part, or multiple pieces. For example in the case of processing `.html` files, you might want to return just one item in the array by combining all scripts, but for `.md` file where each JavaScript block might be independent, you can return multiple items.
+It's up to the plugin to decide if it needs to return just one part of the non-JavaScript file or multiple pieces. For example in the case of processing `.html` files, you might want to return just one item in the array by combining all scripts. However, for `.md` files, you can return multiple items because each JavaScript block might be independent.
 
 **The `postprocess` method** takes a two-dimensional array of arrays of lint messages and the filename. Each item in the input array corresponds to the part that was returned from the `preprocess` method. The `postprocess` method must adjust the locations of all errors to correspond to locations in the original, unprocessed code, and aggregate them into a single flat array and return it.
 
-Reported problems have the following location information:
+Reported problems have the following location information in each lint message:
 
 ```typescript
-{
-    line: number,
-    column: number,
+type LintMessage = {
+
+  /// The 1-based line number where the message occurs.
+  line: number;
+
+   /// The 1-based column number where the message occurs.
+  column: number;
+
+  /// The 1-based line number of the end location.
+  endLine: number;
+
+  /// The 1-based column number of the end location.
+  endColumn: number;
+
+  /// If `true`, this is a fatal error.
+  fatal: boolean;
+
+  /// Information for an autofix.
+  fix: Fix;
+
+  /// The error message.
+  message: string;
+
+  /// The ID of the rule which generated the message, or `null` if not applicable.
+  ruleId: string | null;
+
+  /// The severity of the message.
+  severity: 0 | 1 | 2;
+
+  /// Information for suggestions.
+  suggestions?: Suggestion[];
+};
+
+type Fix = {
+    range: [number, number];
+    text: string;
+}
 
-    endLine?: number,
-    endColumn?: number
+type Suggestion = {
+    desc?: string;
+    messageId?: string;
+    fix: Fix;
 }
+
 ```
 
-By default, ESLint will not perform autofixes when a processor is used, even when the `--fix` flag is enabled on the command line. To allow ESLint to autofix code when using your processor, you should take the following additional steps:
+By default, ESLint does not perform autofixes when a custom processor is used, even when the `--fix` flag is enabled on the command line. To allow ESLint to autofix code when using your processor, you should take the following additional steps:
 
-1. Update the `postprocess` method to additionally transform the `fix` property of reported problems. All autofixable problems will have a `fix` property, which is an object with the following schema:
+1. Update the `postprocess` method to additionally transform the `fix` property of reported problems. All autofixable problems have a `fix` property, which is an object with the following schema:
 
-    ```js
+    ```typescript
     {
         range: [number, number],
         text: string
@@ -81,8 +119,7 @@ By default, ESLint will not perform autofixes when a processor is used, even whe
 
 2. Add a `supportsAutofix: true` property to the processor.
 
-You can have both rules and processors in a single plugin. You can also have multiple processors in one plugin.
-To support multiple extensions, add each one to the `processors` element and point them to the same object.
+You can have both rules and custom processors in a single plugin. You can also have multiple processors in one plugin. To support multiple extensions, add each one to the `processors` element and point them to the same object.
 
 ## Specifying Processor in Config Files
 
@@ -102,19 +139,41 @@ See [Specify a Processor](../use/configure/plugins#specify-a-processor) in the P
 
 ## File Extension-named Processor
 
-If a processor name starts with `.`, ESLint handles the processor as a **file extension-named processor** especially and applies the processor to the kind of files automatically. People don't need to specify the file extension-named processors in their config files.
+If a custom processor name starts with `.`, ESLint handles the processor as a **file extension-named processor**. ESLint applies the processor to files with that filename extension automatically. Users don't need to specify the file extension-named processors in their config files.
 
 For example:
 
 ```js
 module.exports = {
     processors: {
         // This processor will be applied to `*.md` files automatically.
-        // Also, people can use this processor as "plugin-id/.md" explicitly.
+        // Also, you can use this processor as "plugin-id/.md" explicitly.
         ".md": {
             preprocess(text, filename) { /* ... */ },
             postprocess(messageLists, filename) { /* ... */ }
         }
+        // This processor will not be applied to any files automatically.
+        // To use this processor, you must explicitly specify it
+        // in your configuration as "plugin-id/markdown".
+       "markdown": {
+            preprocess(text, filename) { /* ... */ },
+            postprocess(messageLists, filename) { /* ... */ }
+        }
+    }
+}
+```
+
+You can also use the same custom processor with multiple filename extensions. The following example shows using the same processor for both `.md` and `.mdx` files:
+
+```js
+const myCustomProcessor = { /* processor methods */ };
+
+module.exports = {
+    // The same custom processor is applied to both
+    // `.md` and `.mdx` files.
+    processors: {
+        ".md": myCustomProcessor,
+        ".mdx": myCustomProcessor
     }
 }
 ```