Documentation: Revise explanation of cdr.conf option 'Unanswered'

ASTERISK-24279 #close
Reported by: Matt Jordan
Review: https://reviewboard.asterisk.org/r/4109/
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Merged revisions 427901 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@427902 f38db490-d61c-443f-a65b-d21fe96a405b