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


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@384641 f38db490-d61c-443f-a65b-d21fe96a405b