Add auth / rate-limiting info.

proposals/2946-spaces-summary.md

@@ -36,6 +36,8 @@ events. It recurses into the children and into their children, etc.
 Note that there is no requirement for any of the rooms to be of have a `type` of
 `m.space`, any room with `m.space.child` events is considered.
 
+This endpoint requires authentication and is not subject to rate-limiting.
+
 Example request:
 
 ```jsonc