Note preset behaviour in `autocreate_auto_join_room_preset` docs (#17150)

changelog.d/17150.doc

+Clarify the state of the created room when using the `autocreate_auto_join_room_preset` config option.
\ No newline at end of file

docs/usage/configuration/config_documentation.md

@@ -2591,6 +2591,11 @@ Possible values for this option are:
 * "trusted_private_chat": an invitation is required to join this room and the invitee is
   assigned a power level of 100 upon joining the room.
 
+Each preset will set up a room in the same manner as if it were provided as the `preset` parameter when
+calling the
+[`POST /_matrix/client/v3/createRoom`](https://spec.matrix.org/latest/client-server-api/#post_matrixclientv3createroom)
+Client-Server API endpoint.
+
 If a value of "private_chat" or "trusted_private_chat" is used then
 `auto_join_mxid_localpart` must also be configured.