res_pjsip: document use of rewrite_contact in sample conf

Without setting rewrite_contact, an invite to an endpoint
behind NAT will not reach it - unless the endpoint itself
uses STUN or TURN to discover it's public URI.  Thus, the
use of this should be in the sample documentation.

Review: https://reviewboard.asterisk.org/r/4036/
........

Merged revisions 424337 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@424338 f38db490-d61c-443f-a65b-d21fe96a405b