Add docs for new startup probes on Postgres containers
Created by: chrismwendt
Updates CHANGELOGs for https://github.com/sourcegraph/deploy-sourcegraph/pull/4136
Test plan
N/A
Created by: chrismwendt
Updates CHANGELOGs for https://github.com/sourcegraph/deploy-sourcegraph/pull/4136
N/A