Clarify the requirements on the destination block device / file

[randomstuff]

AFAIU, while the source block device has to be a LV snapshot, the destination can be any block device (AFAIU, it can be any file). The documentation does not state that the destination partition needs to be a block device nut all the examples use a LV as a destination. This gives the impression that the target block device needs to be a block device. It might be useful to add a note which explains the requirements wrt the destination file.