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/
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Merged revisions 424337 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 424338 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@424339 65c4cc65-6c06-0410-ace0-fbb531ad65f3