Update documentation for CHANNEL function

Document that you can read/write the 'accountcode' and 'amaflags' on a channel.
........

Merged revisions 384640 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 384641 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@384642 f38db490-d61c-443f-a65b-d21fe96a405b