app_queue: Update sample conf documenation

Updated the queues.conf.sample file to explicitly state which channel queue
variables are propagated to.

ASTERISK-24267
Reported by: Mitch Claborn
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Merged revisions 430126 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 430127 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@430128 f38db490-d61c-443f-a65b-d21fe96a405b