Improve Grafana password documentation
On the Omnibus side gitlab-org/omnibus-gitlab!3732 (merged) was created to note that changing the Grafana password process has changed. While not directly related, it exposed that the documentation of the chart side needs to be improved in this regard.
We need to add to the documentation the process for an end user to rotate the password for Grafana (expected that this would be placed in
secrets.md in the Grafana section). There should also be a reference from the Grafana section in
globals.md so that the procedure can more easily be found.