Spec for MSC4335: M_USER_LIMIT_EXCEEDED error code

changelogs/client_server/newsfragments/2315.feature

@@ -0,0 +1 @@
+Add `M_USER_LIMIT_EXCEEDED` common error code, as per [MSC4335](https://github.com/matrix-org/matrix-spec-proposals/pull/4335).

content/client-server-api/_index.md

@@ -147,6 +147,37 @@ state (e.g.: sending messages, account data, etc) and not routes which
 only read state (e.g.: [`/sync`](#get_matrixclientv3sync),
 [`/user/{userId}/account_data/{type}`](#get_matrixclientv3useruseridaccount_datatype), etc).
 
+`M_USER_LIMIT_EXCEEDED`
+: {{% added-in v="1.18" %}} The request cannot be completed because the user has
+exceeded (or the request would cause them to exceed) a limit associated with
+their account. For example, a user may have reached their allocated storage
+quota, reached a maximum number of allowed rooms, devices, or other
+account-scoped resources, or exceeded usage limits for specific features.
+
+: The error response MUST have an `info_uri` field (string), which is a URI
+that the client can present to the user to provide more context on the
+encountered limit and, if applicable, guidance on how to increase the limit.
+The homeserver MAY return different values for `info_uri` depending on the type
+of limit reached.
+
+: The error response MAY include a `can_upgrade` field (boolean, default `false`).
+If `true`, it indicates that the specific limit encountered can be increased,
+for example by upgrading the user's account tier. If absent or `false`, the
+limit is a hard limit that cannot be increased.
+
+: The HTTP status code will depend on depend on the particular endpoint.
+
+: Example response:
+
+  ```json
+  {
+    "errcode": "M_USER_LIMIT_EXCEEDED",
+    "error": "You have exceeded your storage quota of 10GB",
+    "info_uri": "https://example.com/homeserver/about?limit_type=quota",
+    "can_upgrade": true
+  }
+  ```
+
 `M_UNKNOWN`
 : An unknown error has occurred.