Clarify OWNERS requirement in //testing/buildbot/

Ideally we'd be able to enforce this policy in code. But many folk have
been grandfathered into being able to approve such test additions, so
it'd be difficult to rectify that.

But in a pure starlark world we'd be able to encorce this like we do
for new builders on the CQ today.

Clarifying the comment in this OWNERS file is an attempt at a stop-gap
until that's possible.

Bug: None
Change-Id: I0150cf7b9ac00277b39ecd4578fa3919a37ce50b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3724102
Reviewed-by: Erik Staab <[email protected]>
Commit-Queue: Ben Pastene <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1018274}

testing/buildbot/OWNERS

@@ -2,7 +2,7 @@
 # run tests and understand the implications of changing these files.
 
 # If you're making changes that affect CQ builders (like adding new tests or
-# compile targets) please get a review from someone in //infra/OWNERS.
+# compile targets) you need a review from a Trooper in //infra/OWNERS.
 file://infra/OWNERS
 
 [email protected]