Follow-up from "Adds crossplane CI/CD docs"

The following discussions from !27137 (merged) should be addressed:

• @eread started a discussion:

  You can check the default
  [values.yaml](https://github.com/crossplane/crossplane/blob/master/cluster/charts/crossplane/values.yaml.tmpl)
  we set for this chart. Note that if you don't specify a specific `clusterStack`, GitLab
  defaults to `gcp`.

  Small edit.

• @eread started a discussion:

  available configuration options. Note that this link points to the documentation for the current development release, which may differ from the version you have installed.

  Let's assume the reader knows how to call up previous versions of files.

  Is it easy for someone to know which branch or tag is relevant to them?

• @eread started a discussion:

  > [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/35675) in GitLab 12.9.