Point out that -F probably does not do what the user expects.

Users attempting to create images from mtree METALOG files created by
installworld often use -F when they should be passing the METALOG file
in place of a directory. This is often produces difficult to debug
error reports.

Reviewed by:	emaste
MFC after:	1 week
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D10038

usr.sbin/makefs/makefs.8

@@ -35,7 +35,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd June 14, 2016
+.Dd April 6, 2017
 .Dt MAKEFS 8
 .Os
 .Sh NAME
@@ -116,6 +116,12 @@ set in
 .Ar debug-mask .
 XXX: document these
 .It Fl F Ar mtree-specfile
+.Em This is almost certainly not the option you are looking for.
+To create an image from a list of files in an mtree format manifest,
+specify it as the last argument on the commandline, not as a the
+argument to
+.Fl F .
+.Pp
 Use
 .Ar mtree-specfile
 as an