Improve `getRestHandlerWrapper` JavaDocs (#34376)

Questions on how to work with `ActionPlugin#getRestHandlerWrapper()`
come up in discuss forums all the time. This change adds an example
to the javadoc how this method should/could be used.

server/src/main/java/org/elasticsearch/plugins/ActionPlugin.java

@@ -103,6 +103,22 @@ public interface ActionPlugin {
 
     /**
      * Returns a function used to wrap each rest request before handling the request.
+     * The returned {@link UnaryOperator} is called for every incoming rest request and receives
+     * the original rest handler as it's input. This allows adding arbitrary functionality around
+     * rest request handlers to do for instance logging or authentication.
+     * A simple example of how to only allow GET request is here:
+     * <pre>
+     * {@code
+     *    UnaryOperator<RestHandler> getRestHandlerWrapper(ThreadContext threadContext) {
+     *      return originalHandler -> (RestHandler) (request, channel, client) -> {
+     *        if (request.method() != Method.GET) {
+     *          throw new IllegalStateException("only GET requests are allowed");
+     *        }
+     *        originalHandler.handleRequest(request, channel, client);
+     *      };
+     *    }
+     * }
+     * </pre>
      *
      * Note: Only one installed plugin may implement a rest wrapper.
      */