Also update comments that'd gotten unnecessarily verbose over
ten years of language development.

Bug: v8:8015
Change-Id: I6688ce22e4aa92f66f937159d890b9922f109d43
Reviewed-on: https://chromium-review.googlesource.com/1180357Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55288}