rtpsession: Document the dtmf messages a bit more
rtpsession: Document the dtmf messages a bit more

kakaroto/farstream,tieto/farstream,tieto/farstream,pexip/farstream,pexip/farstream,shadeslayer/farstream,shadeslayer/farstream,tieto/farstream,pexip/farstream,shadeslayer/farstream,tieto/farstream,kakaroto/farstream,tieto/farstream,kakaroto/farstream,shadeslayer/farstream,pexip/farstream,kakaroto/farstream