remove an XXX comment

 - we can't use ast_true here because non-empty strings would no longer be
   evaluated as true
document the return values of pbx_checkcondition() in doxygen format


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@25411 65c4cc65-6c06-0410-ace0-fbb531ad65f3