Add note about AS API.

data/api/client-server/definitions/client_event_without_room_id.yaml

@@ -127,7 +127,8 @@ properties:
           It is recommended that homeservers SHOULD populate this property
           wherever practical, but they MAY omit it if necessary (for example,
           if calculating the value is expensive, servers might choose to only
-          implement it in encrypted rooms).
+          implement it in encrypted rooms). The property is *not* normally populated
+          in the application service API (where there is no clear definition of "requesting user").
         type: string
         example: join
         x-addedInMatrixVersion: "1.11"