Clean up the "offered_media" code

Clean up the "offered_media" code
- Add variable for number of known media streams instead of hardcoding in definition of sip_pvt
- Rename "text" to "codecs" - beacuse it's what it is
- Add documentation for future developers so that we make sure that we define new sdp media types
  for SRTP-variants

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@216883 65c4cc65-6c06-0410-ace0-fbb531ad65f3