rc-update: improve usage message accuracy and consistency

The old message did not indicate that the runlevel argument was optional
for add and del or that it could be used with show.

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>

src/rc/rc-update.c

@@ -195,10 +195,10 @@ show(RC_STRINGLIST *runlevels, bool verbose)
 }
 
 #include "_usage.h"
-#define usagestring ""							      \
-	"Usage: rc-update [options] add service <runlevel>\n"		      \
-	"       rc-update [options] del service <runlevel>\n"		      \
-	"       rc-update [options] show"
+#define usagestring ""							\
+	"Usage: rc-update [options] add <service> [<runlevel>...]\n"	\
+	"   or: rc-update [options] del <service> [<runlevel>...]\n"	\
+	"   or: rc-update [options] [show [<runlevel>...]]"
 #define getoptstring "su" getoptstring_COMMON
 static const struct option longopts[] = {
 	{ "stack",           0, NULL, 's' },