Updating the "sourcing in-band text tracks" section.

Includes new information from the inband text tracks CG for how to map
diverse types of text tracks to TextTrack objects.
Closes https://www.w3.org/Bugs/Public/show_bug.cgi?id=24997
Sourcing of attibute values for multitrack a/v

Expand on the sourcing of multitrack audio and video track attribute
values for id, kind, label and language as input from the inband text
tracks CG.
Closes https://www.w3.org/Bugs/Public/show_bug.cgi?id=24986