add a bit of documentation on what the stun code in rtp.c does

(which is very little, at the moment).

Eventually, when the functionality is extended, the changes can be merged
back to 1.4. At the moment this is pointless.

Note, this change is whitespace only.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@74571 65c4cc65-6c06-0410-ace0-fbb531ad65f3