Update outdated comments and address TODO about kDataIndex.

Change-Id: Ibf6e91857c0430cb9a598c98933db63ac42a1d1f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3189197
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Patrick Thier <pthier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77123}