update comments

  - note Delta bit requirements
  - fix typo
  - remove TODOs around ideas we decided wouldn't work

Change-Id: I0e40abe3a4bc7089ffd773daccecef8f827ae341
Reviewed-on: https://skia-review.googlesource.com/c/eskia/+/239193
Reviewed-by: Mike Klein <mtklein@google.com>
1 file changed