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.

MFC after: 1 week
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