Add a note that ThirdPartyBlockingEnabled is always disabled in AD domains.

Requested in b/117660113.

BUG=none

Change-Id: I31b8b22a3d504c78c25f0cd895e29c9da6f8db35
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1584329
Commit-Queue: Julian Pastarmov <pastarmovj@chromium.org>
Reviewed-by: Lutz Justen <ljusten@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658535}