Add information about limitations of new codec support in channel drivers.

(issue ASTERISK-18680)
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Merged revisions 341094 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@341096 65c4cc65-6c06-0410-ace0-fbb531ad65f3