Also adds some comments to vk_format_utils.h.

Bug: angleproject:3372
Change-Id: I529b9b189e4cdfd400c3c981a47139727d9954ab
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1565062
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>