MFC r314924:

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.