Update note in sip.conf.sample.

Update note in sip.conf.sample about externip and externhost with STUN.

(closes issue #16323)
Reported by: klaus3000
Patches:
      sip.conf.sample-patch.txt uploaded by klaus3000 (license 65)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@268988 65c4cc65-6c06-0410-ace0-fbb531ad65f3