Correct two flaws in sip.conf.sample related to AST-2011-013.

Correct two flaws in sip.conf.sample related to AST-2011-013.

* The sample file listed *two* values for the 'nat' option as being the default.
  Only 'force_rport' is the default.

* The warning about having differing 'nat' settings confusingly referred to both
  peers and users.
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Merged revisions 348515 from http://svn.asterisk.org/svn/asterisk/branches/1.6.2
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Merged revisions 348516 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 348517 from http://svn.asterisk.org/svn/asterisk/branches/10

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@348518 65c4cc65-6c06-0410-ace0-fbb531ad65f3