Add comments to describe some builtins

Bug: v8:11879
Change-Id: Ia0047b701e77b6e766984de527e817874d95f38e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2989102


Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Shu-yu Guo <syg@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75488}