[DOCS] Format the authenticate API (elastic/x-pack-elasticsearch#2572)

Original commit: elastic/x-pack-elasticsearch@bc486dc6be

docs/en/rest-api/security/authenticate.asciidoc

@@ -4,9 +4,24 @@
 
 The Authenticate API enables you to submit a request with a basic auth header to
 authenticate a user and retrieve information about the authenticated user.
-Returns a 401 status code if the user cannot be authenticated.
 
-To authenticate a user, submit a GET request to the `_xpack/security/_authenticate` endpoint:
+
+==== Request
+
+`GET _xpack/security/_authenticate`
+
+
+==== Description
+
+A successful call returns a JSON structure that shows what roles are assigned
+to the user as well as any assigned metadata.
+
+If the user cannot be authenticated, this API returns a 401 status code.
+
+==== Examples
+
+To authenticate a user, submit a GET request to the
+`_xpack/security/_authenticate` endpoint:
 
 [source,js]
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@@ -14,8 +29,7 @@ GET _xpack/security/_authenticate
 --------------------------------------------------
 // CONSOLE
 
-A successful call returns a JSON structure that shows what roles are assigned
+The following example output provides information about the "rdeniro" user:
-to the user as well as any assigned metadata.
 
 [source,js]
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