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

Includes a link to the Sourcing inband text tracks for mapping
diverse types of text tracks to TextTrack objects.
Closes https://www.w3.org/Bugs/Public/show_bug.cgi?id=24997

Cherry picked from c3a32873e10c45fc948541ca229914d4cf07cdee
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 and reference the Sourcing
in-band media resource tracks spec for that.
Closes https://www.w3.org/Bugs/Public/show_bug.cgi?id=24986

Cherry-picked from 792b76480c731e69327d997b40097d3631b61b64