Further extend Vault docs (and other cred mgrs) to demonstrate credential usage

[vito]

see https://github.com/concourse/docs/pull/99

[sqlsolver]

Additionally, I didn't see a mention in the docs of having to set the parent directory for Concourse, e.g. vault secrets enable -path=concourse kv and that was a necessary pre-req to using ((my-secret)) in a pipeline successfully.