Add a bit of doxygen documentation for app_voicemail.

(issue #12326)
Reported by: travishein
Patches:
      app_voicemail_code_documentation.patch uploaded by travishein (license 385)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@111773 65c4cc65-6c06-0410-ace0-fbb531ad65f3