document kvm_open*() support for live vkernels

lib/libkvm/kvm_open.3

@@ -37,7 +37,7 @@
 .\" $FreeBSD: src/lib/libkvm/kvm_open.3,v 1.5.2.6 2001/12/17 10:08:30 ru Exp $
 .\" $DragonFly: src/lib/libkvm/kvm_open.3,v 1.4 2006/05/26 19:39:38 swildner Exp $
 .\"
-.Dd April 19, 1994
+.Dd May 5, 2009
 .Dt KVM_OPEN 3
 .Os
 .Sh NAME
@@ -64,8 +64,8 @@ and
 return a descriptor used to access kernel virtual memory
 via the
 .Xr kvm 3
-library routines.  Both active kernels and crash dumps are accessible
-through this interface.
+library routines.  Both active kernels (including vkernels) and crash
+dumps are accessible through this interface.
 .Pp
 .Fa execfile
 is the executable image of the kernel being examined.
@@ -77,7 +77,8 @@ as determined from
 .Xr getbootfile 3 .
 .Pp
 .Fa corefile
-is the kernel memory device file.  It can be either /dev/mem
+is the kernel memory device file.  It can be /dev/mem, the path to
+the procfs mem file for a running vkernel (i.e. /proc/$pid/mem)
 or a crash dump core generated by
 .Xr savecore 8 .
 If