Update queues.conf.sample documentation.

Update queues.conf.sample documentation.

Update the documentation surrounding the use of MONITOR_EXEC to make it more clear
that it can be used for both Monitor() and MixMonitor() usage.

(closes issue ASTERISK-17413)
Reported by: David Woolley
Patches:
     issue18817_mixmonitor_queues_doc.diff by Michael L. Young (License #5026)
........

Merged revisions 346472 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 346473 from http://svn.asterisk.org/svn/asterisk/branches/10

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@346474 65c4cc65-6c06-0410-ace0-fbb531ad65f3