Tweak docs wording

docs/usage/configuration/config_documentation.md

@@ -3815,7 +3815,7 @@ Sub-options for this setting include:
 * `system_mxid_display_name`: set the display name of the "notices" user
 * `system_mxid_avatar_url`: set the avatar for the "notices" user
 * `room_name`: set the room name of the server notices room
-* `auto_join`: the user will be automatically joined to the room instead of being invited.
+* `auto_join`: boolean. If true, the user will be automatically joined to the room instead of being invited.
   Defaults to false. _Added in Synapse 1.98.0._
 Example configuration:
 ```yaml