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

[brooks]

Summary

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.

[emaste]

Sounds good to me, having been bitten by exactly this issue. If we knew of a use case for -F we could allude to it (e.g. not the option you are looking for, unless ...), but I'm not sure where one would actually want -F.