Reword to remove confusion between su(1)'s "-c" option and that of the shell

being invoked.  Add example that clarifies usage of "-c".

PR: 6859

usr.bin/su/su.1

@@ -30,7 +30,7 @@
 .\" SUCH DAMAGE.
 .\"
 .\"	@(#)su.1	8.2 (Berkeley) 4/18/94
-.\"	$Id: su.1,v 1.12 1997/10/27 22:05:05 guido Exp $
+.\"	$Id: su.1,v 1.13 1998/05/25 03:34:52 steve Exp $
 .\"
 .\" this is for hilit19's braindeadness: "
 .Dd April 18, 1994
@@ -144,13 +144,7 @@ overrides any previous ones.
 If the optional
 .Ar args
 are provided on the command line, they are passed to the login shell of
-the target login.  This allows it to pass arbitrary commands via
-the
-.Fl c
-option as understood by most shells.  Note that
-.Fl c
-usually expects a single argument only; you have to quote it when
-passing multiple words.
+the target login.
 .Pp
 Only users who are a member of group 0 (normally
 .Dq wheel )
@@ -204,7 +198,20 @@ as user
 You will be asked for man's password unless your real UID is 0.
 .It Li "su man -c 'catman /usr/share/man /usr/local/man /usr/X11R6/man'"
 Same as above, but the target command constitutes of more than a
-single word.
+single word and hence is quoted for use with the 
+.Fl c
+option being passed to the shell.  (Most shells expect the argument to
+.Fl c
+to be a single word).
+.It Li "su -c staff man -c 'catman /usr/share/man /usr/local/man /usr/X11R6/man'"
+Same as above, but the target command is run with the resource limits of 
+the login class 
+.Dq staff .
+Note: in this example, the first 
+.Fl c
+option applies to
+.Nm
+while the second is an argument to the shell being invoked.
 .It Li "su -l foo"
 Pretend a login for user
 .Li foo .