Document `CBORReader`'s limitations and their reasons.

Also resolve an API inconsistency: use Optional in 1 last place.

Bug: 879237
Change-Id: I383cce38e47f06cedff99c05c3b2c1e1c67601f7
Reviewed-on: https://chromium-review.googlesource.com/1222916
Commit-Queue: Chris Palmer <palmer@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#591228}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: be2d8dcbe045805779a972a32bc1433dd782b01a
8 files changed