Actually fix ACLs on Presubmit-BoringSSL-Verified label

Think I figured out the issue. The bot needs label-* permission to add a
label, not labelAs-* (the Commit-Queue label is an exception). And human
users in fact do not need to modify the label, so I've removed their
permission.

Also adds a description that this label is informational only (does not
block submit).

Change-Id: I109092cf82c50cfd1c5ae4dd2e500a03047b561a
Reviewed-on: https://boringssl-review.googlesource.com/c/All-Projects/+/91427
Reviewed-by: Adam Langley <agl@google.com>