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
Sponsored by:	Dell EMC Isilon

sbin/devfs/devfs.c

@@ -225,6 +225,8 @@ void
 usage(void)
 {
 
-	fprintf(stderr, "usage: devfs rule|ruleset arguments\n");
+	fprintf(stderr, "usage: %s\n%s\n",
+	    "\tdevfs [-m mount-point] [-s ruleset] rule ...",
+	    "\tdevfs [-m mount-point] ruleset ...");
 	exit(1);
 }