Add note on how to use the account data event

specification/modules/dm.rst

@@ -38,7 +38,8 @@ Client behaviour
 ----------------
 The invitee's client may use the ``is_direct`` flag to automatically mark the
 room as a direct message but this is not required: it may for example, prompt
-the user, ignore the flag altogether.
+the user, ignore the flag altogether. To do this, it stores this event in
+account data using the ``account_data`` API: see `Client Config`_.
 
 The inviter's client should set the ``is_direct`` flag to  ``createRoom``
 whenever the flow the user has followed is one where their intention is to