Update documentation about trybot/CI builder mirroring and the CQ.

The documentation for adding builders and the CQ didn't explain why
it was important that we have mirrored CI bots for every trybot and
vice versa, so this CL attempts to fix that, and make other related
improvements at the same time.

It also updates the doc to note that estaab@ is responsible for
approving new builders in the CQ, not dpranke@.

And, it updates a few lingering references to "buildbot masters"
to "builder groups".

Change-Id: I27f1a9d59ab8edf5cccd58ffc1815e2e88e52f1b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2898164
Commit-Queue: Dirk Pranke <dpranke@google.com>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#883533}