res/res_pjsip_pubsub: Note that 'dialog' is also a valid event type for RLS

In addition to specifying lists of 'presence' and 'message-summary',
users can also create lists of type 'dialog'. These should be treated in
the same fashion as 'presence'.

Change-Id: I583bb69cd9f88b0b29bf09ddaddeac4e84189f6e