update docs for `-force`

[lucaudill]

I was recently working on a feature that uses unsquashfs (conversation here), and ran into some confusion over the -force option. The option is documented as overwriting existing files, but I found that in the case where it is combined with -d and the destination exists but is empty, the archive is unpacked to the directory without overwriting. This may be an edge case, but I think documenting this behavior would be helpful.