Document that tmpl_secret is not for putting secrets in files.
Document that tmpl_secret is not for putting secrets in files.

Also clean up wording of Prometheus admin flags.

ddfreyne/prometheus-docs,chosenken/docs,ddfreyne/prometheus-docs,chosenken/docs,prometheus/docs,ddfreyne/prometheus-docs,ddfreyne/prometheus-docs,chosenken/docs,prometheus/docs,prometheus/docs,chosenken/docs