Add missing description of the PARKINGLOT variable in XML documentation.

(closes issue #16743)
Reported by: snuffy
Patches: 
      parkingdoc.diff uploaded by snuffy (license 35)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@249925 65c4cc65-6c06-0410-ace0-fbb531ad65f3