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