Better explain why internalized strings have to be in old space. The
scavenger doesn't need to iterate and update references in the string
table and the stub cache.

Change-Id: I93c3e0b743f85fbf4de2ad877f3667abb2e0ae53
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2949101
Auto-Submit: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75052}