Also document them.

BUG=v8:5830
R=marja@chromium.org

Change-Id: I86e71c8e24e4b7f3032e0f2365cf72575b1eb413
Reviewed-on: https://chromium-review.googlesource.com/518155Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45595}