Update documentation (see jestjs#12260)

This PR specifies how to use the filtering function CLI option.

website/versioned_docs/version-28.0/CLI.md

@@ -204,7 +204,18 @@ Alias: `-e`. Use this flag to show full diffs and errors instead of a patch.
 
 ### `--filter=<file>`
 
-Path to a module exporting a filtering function. This method receives a list of tests which can be manipulated to exclude tests from running. Especially useful when used in conjunction with a testing infrastructure to filter known broken.
+Path to a module exporting a filtering function. This asynchronous function receives a list of test paths which can be manipulated to exclude tests from running by returning an object with the "filtered" property. Especially useful when used in conjunction with a testing infrastructure to filter known broken, e.g.
+
+```javascript
+// my-filter.js
+module.exports = (testPaths) => {
+  const allowedPaths = testPaths.filter(filteringFunction) // ["path1.spec.js", "path2.spec.js", etc]
+  return {
+      filtered : allowedPaths,
+   }
+}
+```
+
 
 ### `--findRelatedTests <spaceSeparatedListOfSourceFiles>`