Add `account_data` to left rooms in /sync

Fixes https://github.com/matrix-org/matrix-doc/issues/1392

api/client-server/sync.yaml

@@ -227,6 +227,14 @@ paths:
                             room up to the point when the user left.
                           allOf:
                             - $ref: "definitions/timeline_batch.yaml"
+                        account_data:
+                          title: Account Data
+                          type: object
+                          description: |-
+                            The private data that this user has attached to
+                            this room.
+                          allOf:
+                          - $ref: "definitions/event_batch.yaml"
               presence:
                 title: Presence
                 type: object