This adds some documentation why concurrently emitting code, patching
the jump table, and executing the jump table is safe.

R=ahaas@chromium.org
CC=​mstarzinger@chromium.org, joey.gouly@arm.com

Bug: v8:9477
Change-Id: Ibe295d538a1a330c6b1d94eb1f514d1078020754
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1738856
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63137}