Update prep_tarball with new documentation files on the Asterisk wiki

Update prep_tarball with new documentation files on the Asterisk wiki

This will now pull both a command reference for the version being prepared,
as well as an Admin Guide that applies to all versions of Asterisk.

(issue ASTERISK-22439)
Reported by: Olle Johansson
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Merged revisions 399351 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 399373 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@399376 65c4cc65-6c06-0410-ace0-fbb531ad65f3