Update new Local channel documentation.

Update new Local channel documentation.

The original reporter, Kobaz, of an issue with a Local channel that inspired the
Local channel documentation provided some tweaks to the documentation after testing
what I had written. Hopefully anything that was vague or unclear has been cleaned
up by these changes.

(closes issue #16963)
Reported by: kobaz
Patches:
      localchannel-2.txt uploaded by kobaz (license 834)
Tested by: kobaz, lmadsen

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@253252 65c4cc65-6c06-0410-ace0-fbb531ad65f3