Clean up from the 4.x era.

In an example of boot command:
- rename wd(4) IDE disk drives name to ad(4) for the time being.
- update the used kernel path "/kernel" to the current default.
[It still worked occasionally by looking into the /boot/kernel directory,
so the resulting path was "/boot//kernel/kernel", with two slashes.]

Bump .Dd for this and previous changes.

MFC after:	1 week

sys/boot/common/loader.8

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd February 15, 2009
+.Dd December 27, 2011
 .Dt LOADER 8
 .Os
 .Sh NAME
@@ -926,10 +926,10 @@ autoboot 5
 .Pp
 Set the disk unit of the root device to 2, and then boot.
 This would be needed in a system with two IDE disks,
-with the second IDE disk hardwired to wd2 instead of wd1.
+with the second IDE disk hardwired to ad2 instead of ad1.
 .Bd -literal -offset indent
 set root_disk_unit=2
-boot /kernel
+boot /boot/kernel/kernel
 .Ed
 .Pp
 See also: