Add M_USER_DEACTIVATED to list of error codes (#2234)

Spec PR for [MSC 2181](https://github.com/matrix-org/matrix-doc/pull/2181).

Adds the `M_USER_DEACTIVATED` error code and a short description to the client-server API.

api/client-server/login.yaml

@@ -194,10 +194,13 @@ paths:
             "$ref": "definitions/errors/error.yaml"
         403:
           description: |-
-            The login attempt failed. For example, the password may have been incorrect.
+            The login attempt failed. This can include one of the following error codes:
+              * ``M_FORBIDDEN``: The provided authentication data was incorrect.
+              * ``M_USER_DEACTIVATED``: The user has been deactivated.
           examples:
             application/json: {
-              "errcode": "M_FORBIDDEN"}
+              "errcode": "M_FORBIDDEN"
+            }
           schema:
             "$ref": "definitions/errors/error.yaml"
         429:

changelogs/client_server/newsfragments/2234.feature

@@ -0,0 +1 @@
+Add ``M_USER_DEACTIVATED`` error code.

specification/client_server_api.rst

@@ -150,6 +150,10 @@ Other error codes the client might encounter are:
 :``M_UNAUTHORIZED``:
   The request was not correctly authorized. Usually due to login failures.
 
+:``M_USER_DEACTIVATED``:
+  The user ID associated with the request has been deactivated. Typically for
+  endpoints that prove authentication, such as ``/login``.
+
 :``M_USER_IN_USE``:
   Encountered when trying to register a user ID which has been taken.