rc(8): document how to reload rc's configuration

The fact that rc(8) only reads its configuration once is in the man
page, but how to trigger a reload is only in the code.

Discovered while trying to make cloud-init disable and stop syslogd.

Thanks to RhodiumToad for providing the words.

Reviewed by: debdrup, grahamperrin
Approved by: grahamperrin, imp
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D40329