Update LintMessage docs

This updates existing documented properties to match the updated `LintMessage`
type and implementations.

`messageId`, `nodeType`, and `suppressions` remain undocumented.

docs/src/extend/custom-processors.md

@@ -59,19 +59,19 @@ Reported problems have the following location information in each lint message:
 type LintMessage = {
 
   /// The 1-based line number where the message occurs.
-  line: number;
+  line?: number;
 
    /// The 1-based column number where the message occurs.
-  column: number;
+  column?: number;
 
   /// The 1-based line number of the end location.
-  endLine: number;
+  endLine?: number;
 
   /// The 1-based column number of the end location.
-  endColumn: number;
+  endColumn?: number;
 
   /// If `true`, this is a fatal error.
-  fatal: boolean;
+  fatal?: boolean;
 
   /// Information for an autofix.
   fix: Fix;