Merge some channel.h doxygen updates from team/russell/chan_refcount

This was mostly to note whether a channel needed to be locked or not before
calling these functions.  However, I added some other things, too.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@90139 65c4cc65-6c06-0410-ace0-fbb531ad65f3