Add a hint that "device ctl" kind of depends on "device iscsi".

It actually doesn't - "device ctl" automatically pulls in ICL, which
would normally be a part of iscsi.ko.  However, doing it that way makes
iscsi.ko unloadable, and building ctl.ko without iscsi.ko (using
MODULES_OVERRIDE) results in ctl.ko that is unloadable, due to missing
symbols that would be resolved to iscsi.ko.  And since the symbols
are named "icl_whatever", it's not obvious that it's iscsi.ko that's
required.

If there is a better way - let me know.

MFC after:	1 month
Sponsored by:	The FreeBSD Foundation

share/man/man4/ctl.4

@@ -23,7 +23,7 @@
 .\" SUCH DAMAGE.
 .\"
 .\" $FreeBSD$
-.Dd October 23, 2014
+.Dd April 8, 2015
 .Dt CTL 4
 .Os
 .Sh NAME
@@ -34,6 +34,7 @@ To compile this driver into the kernel,
 place the following line in your
 kernel configuration file:
 .Bd -ragged -offset indent
+.Cd "device iscsi"
 .Cd "device ctl"
 .Ed
 .Pp