This improves documentation about some things that came up
in conversation and things that I noticed while working on
those other things. :)

Change-Id: I4f47cec6594f7b331259bea8ed506f5de908d438
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1954386
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65449}