Document the new behavior

docs/integrations/github_actions.md

@@ -32,9 +32,12 @@ We recommend the use of the `@stable` tag, but per version tags also exist if yo
 that. Note that the action's version you select is independent of the version of _Black_
 the action will use.
 
-The version of _Black_ the action will use can be configured via `version`. The action
-defaults to the latest release available on PyPI. Only versions available from PyPI are
-supported, so no commit SHAs or branch names.
+The version of _Black_ the action will use can be configured via `version`. This can be
+any
+[valid version specifier](https://packaging.python.org/en/latest/glossary/#term-Version-Specifier)
+or just the version number if you want an exact version. The action defaults to the
+latest release available on PyPI. Only versions available from PyPI are supported, so no
+commit SHAs or branch names.
 
 You can also configure the arguments passed to _Black_ via `options` (defaults to
 `'--check --diff'`) and `src` (default is `'.'`)
@@ -48,3 +51,15 @@ Here's an example configuration:
     src: "./src"
     version: "21.5b1"
 ```
+
+If you want to match versions covered by Black's
+[stability policy](labels/stability-policy), you can use the compatible release operator
+(`~=`):
+
+```yaml
+- uses: psf/black@stable
+  with:
+    options: "--check --verbose"
+    src: "./src"
+    version: "~= 22.0"
+```