Remove an XXX command suggesting that this truncation should not be conditional,

and also add a more verbose comment explaining why it is only needed in the
case of appending to the string for any curious readers that come along in the
future.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@48103 65c4cc65-6c06-0410-ace0-fbb531ad65f3