monitoring: fix ContainerVariable validate, tweak remote prometheus docs

Created by: bobheadxi

Some adjustments I noticed as I skimmed through this code again as part of https://github.com/sourcegraph/sourcegraph/pull/30997

• monitoring: fix ContainerVariable validation. We were previously not validating that only one of OptionsQuery and Options is set
• doc/dev: tweak sg overwrite advice for remote prometheus. Skips the lengthy Prometheus build in favour of authenticating against a cluster

Test plan

sg run monitoring-generator