Update documentation for 'comebacktoorigin' in featuers.conf.

The documentation for this option did not match the code.  Fix that along with
some minor cleanups to the code along the way.  Document a slight change in
behavior (to something that was previously undocumented) in UPGRADE.txt.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@278425 65c4cc65-6c06-0410-ace0-fbb531ad65f3