[xiph-rtp] Updated vorbis document

[Luca Barbato]

I should have eventually addressed all the formal issues with the
previous spec.

Please review it again for possible issues:

http://svn.xiph.org/trunk/vorbis/doc/draft-ietf-avt-rtp-vorbis-01.xml
http://svn.xiph.org/trunk/vorbis/doc/draft-ietf-avt-rtp-vorbis-01.txt

Changes:

Split iana and sdp sections, minor wording changes, reference cleanup.

lu