res_pjsip: enhance documentation for mailboxes options, for both endpoints and aors

Made documentation more explicit as to the use of the both options.

(issue ASTERISK-23071)
(closes issue ASTERISK-23071)
Reported by: Matt Jordan
........

Merged revisions 405777 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@405778 65c4cc65-6c06-0410-ace0-fbb531ad65f3