MFC r314924:
sbin/devfs: clarify usage
- Note existence of -m option.
- Note that -s applies to rule keyword, only, by adding usage text
specifically for the `rule` and `ruleset` keywords.
Don't go into any further detail in usage(..) -- it's best that one
reads the manpage to get a better idea of how things work as there are
a number of different option-specific keywords and arguments, as well
as some rule grammar.
git-svn-id: svn://svn.freebsd.org/base/stable/10@315355
ccf9f872-aa2e-dd11-9fc8-
001c23d0bc1f