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


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@399351 f38db490-d61c-443f-a65b-d21fe96a405b