Mention supported keys

specification/modules/search.rst

@@ -11,11 +11,22 @@ Client behaviour
 {{search_http_api}}
 
 Search Categories
-~~~~~~~~~~~~~~~~~
+-----------------
 
 The search API allows clients to search in different categories of items.
-Currently the only specified category is ``room_events``, which include
-events in rooms the user had joined.
+Currently the only specified category is ``room_events``.
+
+``room_events``
+~~~~~~~~~~~~~~~
+
+This category covers all events in rooms that the user had joined. The search
+is performed on certain keys of certain event types.
+
+The supported keys to search over are:
+
+- ``content.body`` in ``m.room.message``
+- ``content.name`` in ``m.room.name``
+- ``content.topic`` in ``m.room.topic``
 
 Security considerations
 -----------------------