fix: Add extra note about setting POSTGRES_ENABLED=false

Addresses a concern raised in
https://gitlab.com/gitlab-org/gitlab/-/issues/218388 that some users, in
particular users of custom charts, may have the helm value
postgresql.enabled set to true without actually relying on it. For them,
setting POSTGRES_ENABLED=false is the correct way forward, as otherwise
we would install an unwanted database.