Clean up documentation; prevent ref leak on exit

This patch:
 * Cleans up some doxygen
 * Prevents leaking the system level Stasis topics and messages
   on exit (users of valgrind will be happier)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@387633 65c4cc65-6c06-0410-ace0-fbb531ad65f3