Update existing Local channel documentation.

A complete re-write of the Local channel documentation has been performed, with
the existing information from localchannel.txt and localchannel.tex merged in.

(closes issue #16637)
Reported by: kobaz
Patches: 
      localchannel.tex uploaded by lmadsen (license 10)
      localchannel.txt uploaded by lmadsen (license 10)
Tested by: lmadsen, jsmith, mmichelson

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@250609 65c4cc65-6c06-0410-ace0-fbb531ad65f3