Describe M_UNSUPPORTED_ROOM_VERSION on invite and createRoom endpoints

Spec for MSC1866: https://github.com/matrix-org/matrix-doc/pull/1866

api/client-server/create_room.yaml

@@ -244,6 +244,12 @@ paths:
               invalid: for example, the user's ``power_level`` is set below
               that necessary to set the room name (``errcode`` set to
               ``M_INVALID_ROOM_STATE``).
+
+            - The homeserver doesn't support the requested room version, or
+              one or more users being invited to the new room are residents
+              of a homeserver which does not support the requested room version.
+              The ``errcode`` will be ``M_UNSUPPORTED_ROOM_VERSION`` in these
+              cases.
           schema:
             "$ref": "definitions/errors/error.yaml"
       tags:

api/client-server/inviting.yaml

@@ -82,6 +82,20 @@ paths:
               }
           schema:
             type: object
+        400:
+          description: |-
+
+            The request is invalid. A meaningful ``errcode`` and description
+            error text will be returned. Example reasons for rejection include:
+
+            - The request body is malformed (``errcode`` set to ``M_BAD_JSON``
+              or ``M_NOT_JSON``).
+
+            - One or more users being invited to the room are residents of a
+              homeserver which does not support the requested room version. The
+              ``errcode`` will be ``M_UNSUPPORTED_ROOM_VERSION`` in these cases.
+          schema:
+            "$ref": "definitions/errors/error.yaml"
         403:
           description: |-
             You do not have permission to invite the user to the room. A meaningful ``errcode`` and description error text will be returned. Example reasons for rejections are:

changelogs/client_server/newsfragments/1903.feature

@@ -0,0 +1 @@
+Emit ``M_UNSUPPORTED_ROOM_VERSION`` error codes where applicable on ``/createRoom`` and ``/invite`` APIs.