Describe the rate limit error everywhere

Fixes https://github.com/matrix-org/matrix-doc/issues/1153

api/client-server/definitions/errors/error.yaml

@@ -17,7 +17,9 @@ properties:
   errcode:
     type: string
     description: An error code.
+    example: M_UNKNOWN
   error:
     type: string
     description: A human-readable error message.
+    example: An unknown error occurred
 required: ["errcode"]

api/client-server/definitions/errors/rate_limited.yaml

@@ -0,0 +1,26 @@
+# Copyright 2018 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+$ref: error.yaml
+type: object
+description: The rate limit was reached for this request
+properties:
+  errcode:
+    type: string
+    description: The M_LIMIT_EXCEEDED error code
+    example: M_LIMIT_EXCEEDED
+  error:
+    type: string
+    description: A human-readable error message.
+    example: Too many requests
+required: ["errcode"]