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: https://origsvn.digium.com/svn/asterisk/branches/11@430126 65c4cc65-6c06-0410-ace0-fbb531ad65f3