Update documentation for CHANNEL function

Document that you can read/write the 'accountcode' and 'amaflags' on a channel.
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Merged revisions 384640 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 384641 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@384642 65c4cc65-6c06-0410-ace0-fbb531ad65f3