Clarify when we should use PostTask vs PostNonNestableTask.

Change-Id: I2b69dc4d4b6de4079dcab05e8b5c3b1f77cbbe2c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2253852
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68475}