Add some docs for `verify_build_configs`

## Add some docs for `verify_build_configs`

### Description

Since #12491 there is a test for `verify_build_configs`

Its not immediately obvious if you are adding a sandbox how this works, or even that it needs to be added

See here https://github.com/envoyproxy/envoy/pull/12549#issuecomment-690985275 for discussion