External DB docs should link to secrets docs
The advanced external db docs should include a link to the postgres section of our secrets documentation.
So it's clear that the secret needs to be created ahead of time with the postgres password information.
Doc is here: https://gitlab.com/charts/helm.gitlab.io/blob/master/doc/advanced/external-db/README.md
Secrets are here: https://gitlab.com/charts/helm.gitlab.io/blob/master/doc/installation/secrets.md#postgres-password