docs/config: make it explicit that auto_join_rooms can be used for invite-only rooms (#19660)

changelog.d/19660.doc

@@ -0,0 +1 @@
+Update `auto_join_rooms` config documentation to cover requirements for auto-joining invite-only rooms.

docs/usage/configuration/config_documentation.md

@@ -2870,6 +2870,8 @@ enable_3pid_changes: false
 
 By default, any room aliases included in this list will be created as a publicly joinable room when the first user registers for the homeserver. If the room already exists, make certain it is a publicly joinable room, i.e. the join rule of the room must be set to `public`. You can find more options relating to auto-joining rooms below.
 
+Invite-only rooms can also be auto-joined when setting `auto_join_mxid_localpart` to a user who's part of the invite-only rooms.
+
 As Spaces are just rooms under the hood, Space aliases may also be used.
 
 Defaults to `[]`.

schema/synapse-config.schema.yaml

@@ -3227,6 +3227,10 @@ properties:
       options relating to auto-joining rooms below.
 
 
+      Invite-only rooms can also be auto-joined when setting `auto_join_mxid_localpart`
+      to a user who's part of the invite-only rooms.
+
+
       As Spaces are just rooms under the hood, Space aliases may also be used.
     items:
       type: string