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: https://origsvn.digium.com/svn/asterisk/branches/13@424338 65c4cc65-6c06-0410-ace0-fbb531ad65f3

configs/samples/pjsip.conf.sample

@@ -361,6 +361,7 @@
 ;direct_media=no
 ;rtp_symmetric=yes
 ;force_rport=yes
+;rewrite_contact=yes  ; necessary if endpoint does not know/register public ip:port
 ;ice_support=yes   ;This is specific to clients that support NAT traversal
                    ;for media via ICE,STUN,TURN. See the wiki at:
                    ;https://wiki.asterisk.org/wiki/x/D4FHAQ