Add more explicit comments and change some behavior slightly.

Bug: angleproject:3369
Change-Id: I76f9054564e8d33127e8ff8233b317e82e06c7de
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1582760Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>