More draft edits and additions.

Some clean-up from JM's prior edits, as well as
* Additional clarificatino of TOC sequence restrictions (e.g., if
   you need to decode a length, there must be enough bytes in the
   packet for it, etc.).
* Added a summary of TOC sequence restrictions.
* Added a stereo unmixing section.
* Reworked Delay Compensation section into a general Resampling
   section.
* Further cleanups of switching/transitions, including new diagrams
   for all of the normative and recommended non-normative
   transitions.