[heap] Update CodePageCollectionMemoryModificationScope docs

The scope can be (and is) used by compiler threads, i.e. not just by the
main thread; update the comment.

Change-Id: Ifbeb414761cee4ab12dd953f0cd544f35c92fc8a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3516037


Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79445}