Elaborate on POSTGRES_VERSION Auto DevOps setting

doc/topics/autodevops/index.md

@@ -697,7 +697,7 @@ also be customized, and you can easily use a [custom buildpack](#custom-buildpac
 | `POSTGRES_USER`              | The PostgreSQL user; defaults to `user`. Set it to use a custom username.                                                                                                                                                     |
 | `POSTGRES_PASSWORD`          | The PostgreSQL password; defaults to `testing-password`. Set it to use a custom password.                                                                                                                                     |
 | `POSTGRES_DB`                | The PostgreSQL database name; defaults to the value of [`$CI_ENVIRONMENT_SLUG`](../../ci/variables/README.md#predefined-environment-variables). Set it to use a custom database name.                               |
-| `POSTGRES_VERSION`           | The PostgreSQL version; defaults to `9.6.2` |
+| `POSTGRES_VERSION`           | Tag for the [`postgres` Docker image](https://hub.docker.com/_/postgres) to use. Defaults to `9.6.2`. |
 | `BUILDPACK_URL`              | The buildpack's full URL. It can point to either Git repositories or a tarball URL. For Git repositories, it is possible to point to a specific `ref`, for example `https://github.com/heroku/heroku-buildpack-ruby.git#v142` |
 | `SAST_CONFIDENCE_LEVEL`      | The minimum confidence level of security issues you want to be reported; `1` for Low, `2` for Medium, `3` for High; defaults to `3`.|
 | `DEP_SCAN_DISABLE_REMOTE_CHECKS` | Whether remote Dependency Scanning checks are disabled; defaults to `"false"`. Set to `"true"` to disable checks that send data to GitLab central servers. [Read more about remote checks](https://gitlab.com/gitlab-org/security-products/dependency-scanning#remote-checks).|