Document the error code when someone tries to forget a room they are in

api/client-server/leaving.yaml

@@ -99,6 +99,15 @@ paths:
               }
           schema:
             type: object
+        400:
+          description: The user has not left the room
+          examples:
+            application/json: {
+              "errcode": "M_UNKNOWN",
+              "error": "User @example:matrix.org is in room !au1ba7o:matrix.org"
+            }
+          schema:
+            "$ref": "definitions/error.yaml"
         429:
           description: This request was rate-limited.
           schema: