Clarify documentation for VoiceMailMain()'s a() option.

We require box numbers, not names as the documentation implies.
(issue #14740)
Reported by: pj
Patches:
      __20090729-app_voicemail-documentation.patch uploaded by lmadsen (license 10)
Tested by: seanbright, lmadsen


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@221085 65c4cc65-6c06-0410-ace0-fbb531ad65f3