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


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@430126 f38db490-d61c-443f-a65b-d21fe96a405b