Add note about how Authenticate() application with option 'd' works.

(closes issue ASTERISK-17422)
Reported by: Leif Madsen
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Merged revisions 343102 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 343103 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@343104 65c4cc65-6c06-0410-ace0-fbb531ad65f3