Document GET method for retrieving admin bit of user in admin API

Signed-off-by: Olivier Wilkinson (reivilibre) <olivier@librepush.net>

docs/admin_api/user_admin_api.rst

@@ -86,6 +86,25 @@ with a body of:
 including an ``access_token`` of a server admin.
 
 
+Get whether a user is a server administrator or not
+===================================================
+
+
+The api is::
+
+    GET /_synapse/admin/v1/users/<user_id>/admin
+
+including an ``access_token`` of a server admin.
+
+A response body like the following is returned:
+
+.. code:: json
+
+    {
+        "admin": true
+    }
+
+
 Change whether a user is a server administrator or not
 ======================================================