More draft updates.

A number of fixes and additions, including
* Ensure all usage of the word "mode" refer only to the choice of
   SILK/hybrid/CELT, not audio bandwidth, frame size, channel count,
   or anything else.
  There's still a bunch of usage of "mode" in CELT to refer to
   things in the CELTMode struct (e.g., band layout, etc.).
* Document the LSF reordering for silk_NLSF2A_find_poly().
* Document the DC response check during LSF stabilization.
* Fix the excitation scaling to give decoded SILK output in the
   range -1.0...1.0.
* Rewrite the mode-switching section.
  Ironically the title of the section still implies "mode" means
   than just SILK/hybrid/CELT, but I couldn't come up with a
   better one.
* Minor clean-ups to the acknowledgements.