Improve documentation for `/_synapse/admin/v1/rooms/<room_id>/timestamp_to_event` (#16631)

changelog.d/16631.doc

@@ -0,0 +1 @@
+Update parameter information for the `/timestamp_to_event` admin API.

docs/admin_api/rooms.md

@@ -536,7 +536,8 @@ The following query parameters are available:
 
 **Response**
 
-* `event_id` - converted from timestamp
+* `event_id` - The event ID closest to the given timestamp.
+* `origin_server_ts` - The timestamp of the event in milliseconds since the Unix epoch.
 
 # Block Room API
 The Block Room admin API allows server admins to block and unblock rooms,