Document that kvm_open(3) also accepts "/dev/null" as a special

"corefile" argument, to access the running system via sysctl(3)
if possible, thus not requring special setgid privileges.

lib/libkvm/kvm_open.3

@@ -36,7 +36,7 @@
 .\"     @(#)kvm_open.3	8.3 (Berkeley) 4/19/94
 .\" $FreeBSD$
 .\"
-.Dd April 19, 1994
+.Dd January 29, 2004
 .Dt KVM_OPEN 3
 .Os
 .Sh NAME
@@ -78,7 +78,9 @@ as determined from
 .Pp
 The
 .Fa corefile
-argument is the kernel memory device file.  It can be either /dev/mem
+argument is the kernel memory device file.
+It can be either
+.Pa /dev/mem
 or a crash dump core generated by
 .Xr savecore 8 .
 If
@@ -87,7 +89,14 @@ is
 .Dv NULL ,
 the default indicated by
 .Dv _PATH_MEM
-from <paths.h> is used.
+from
+.In paths.h
+is used.
+It can also be set to a special value
+.Pa /dev/null
+by utilities like
+.Xr ps 1
+that do not directly access kernel memory.
 .Pp
 The
 .Fa swapfile
@@ -190,4 +199,6 @@ left little choice.
 .Xr kvm_getprocs 3 ,
 .Xr kvm_nlist 3 ,
 .Xr kvm_read 3 ,
-.Xr kvm_write 3
+.Xr kvm_write 3 ,
+.Xr kmem 4 ,
+.Xr mem 4