Improve getRestHandlerWrapper JavaDocs (elastic#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.

Author: s1monw

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

@@ -103,6 +103,22 @@ default Collection<String> getTaskHeaders() {
 
  /**
  * 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.
  */