[Lookalikes] Update docs to add a warning for invalid accounts

If the email address doesn't have an associated Google account, we can't
CC it on the bugs. Add a warning for this case.

Change-Id: Ic047db9886b999b37f5f1d1db4054f328126a1ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5421829


Auto-Submit: Mustafa Emre Acer <meacer@chromium.org>
Commit-Queue: Joe DeBlasio <jdeblasio@chromium.org>
Reviewed-by: Joe DeBlasio <jdeblasio@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282187}