mptutil: document that disabled/enabled are also supported

This change officially documents the fact that the `volume cache
enabled` and volume cache disabled are also supported.

While here, remove an unnecessary newline at the end of a warn*(3) call.
warn*(3) adds a newline automatically.

MFC after: 1 week

(cherry picked from commit 9a49a3792940497ddf4657f6e5be56d8ef881edc)