* [bre] Flag to control pools based on test sizes

* more docs

This options will have the server start in the user-specifed cgroup. This is useful when using the cgroup features in bazel and thus the server must be started in a user-writable cgroup.

Closes bazelbuild#21618.

PiperOrigin-RevId: 666305298
Change-Id: Ide3bb3f65bd11f4ecc11d6e843926a16ecb41f2f

We have an internal system which tacks an aspect onto all the builds it runs,
but no one really cares about the aspect's outputs, just that with the default
--show_result their normal results are suppressed. This lets us avoid that.

There are a few ways we could have implemented this, I decided to go with the
simplest - non-repeated flag with comma-separated values. Practically, we could
evolve this to allow multiple and do something like we do with --output_group
with adding/subtracting from the existing/default, but we can cross that bridge
when/if we get there.

PiperOrigin-RevId: 603229525
Change-Id: I8b1a387eeb7e67cceaf4f8e821bd29c12b136982