service(8): direct user to rc(8) for a list of valid commands

New users may refer to service(8) to discover how to manage services,
but this manpage does not explain which commands are permitted besides
start/stop (for example, 'enable'). Add a paragraph that directs the
reader to rc(8) to discover this.

While here, add a few examples of common use-cases.

Reported by: Mina Galić <freebsd@igalic.co>
Pull request: https://github.com/freebsd/freebsd-src/pull/1057