kvm(3): Mention that some of the functions use sysctl(3) instead of kmem.

Additionally, because of sysctl(3) use (which is generally good), behaviour
for crash dumps differs slightly from behaviour for live kernels and this
will probably never be fixed entirely, so weaken that claim.

MFC after:	1 week

lib/libkvm/kvm.3

@@ -32,7 +32,7 @@
 .\"     @(#)kvm.3	8.1 (Berkeley) 6/4/93
 .\" $FreeBSD$
 .\"
-.Dd January 29, 2004
+.Dd April 25, 2010
 .Dt KVM 3
 .Os
 .Sh NAME
@@ -46,12 +46,15 @@ The
 library provides a uniform interface for accessing kernel virtual memory
 images, including live systems and crash dumps.
 Access to live systems is via
+.Xr sysctl 3
+for some functions, and
 .Xr mem 4
 and
 .Xr kmem 4
+for other functions,
 while crash dumps can be examined via the core file generated by
 .Xr savecore 8 .
-The interface behaves identically in both cases.
+The interface behaves similarly in both cases.
 Memory can be read and written, kernel symbol addresses can be
 looked up efficiently, and information about user processes can
 be gathered.
@@ -112,5 +115,6 @@ given descriptor.
 .Xr kvm_openfiles 3 ,
 .Xr kvm_read 3 ,
 .Xr kvm_write 3 ,
+.Xr sysctl 3 ,
 .Xr kmem 4 ,
 .Xr mem 4