Add doxygen documentation for slinfactory_destroy(), mainly just noting that

it doesn't free the slinfactory itself.  (This isn't related to a bug, i'm just
looking over random code)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@81999 65c4cc65-6c06-0410-ace0-fbb531ad65f3