docs: fix Configuration, JestPlatform and JestObjectAPI docs for 26 (#…

…9985)

website/versioned_docs/version-26.0/Configuration.md

@@ -1086,6 +1086,7 @@ Example:
 Sort test path alphabetically.
 
 ```js
+// testSequencer.js
 const Sequencer = require('@jest/test-sequencer').default;
 
 class CustomSequencer extends Sequencer {
@@ -1100,6 +1101,14 @@ class CustomSequencer extends Sequencer {
 module.exports = CustomSequencer;
 ```
 
+Use it in your Jest config file like this:
+
+```json
+{
+  "testSequencer": "path/to/testSequencer.js"
+}
+```
+
 ### `testTimeout` [number]
 
 Default: `5000`
@@ -1116,7 +1125,9 @@ This option sets the URL for the jsdom environment. It is reflected in propertie
 
 Default: `real`
 
-Setting this value to `fake` allows the use of fake timers for functions such as `setTimeout`. Fake timers are useful when a piece of code sets a long timeout that we don't want to wait for in a test.
+Setting this value to `legacy` or `fake` allows the use of fake timers for functions such as `setTimeout`. Fake timers are useful when a piece of code sets a long timeout that we don't want to wait for in a test.
+
+If the value is `modern`, [`@sinonjs/fake-timers`](https://github.com/sinonjs/fake-timers) will be used as implementation instead of Jest's own legacy implementation. This will be the default fake implementation in Jest 27.
 
 ### `transform` [object\<string, pathToTransformer | [pathToTransformer, object]>]