docs: Note that zero detection on migration sparsifies image only when discard='unmap...

docs: Note that zero detection on migration sparsifies image only when discard='unmap' is set

The mirroring job clears the destination to ensure that the guest
visible disk contents are identical to the state on the source. The
image itself is kept sparse only when the disk 'discard' option is set
to 'unmap' (Also the disks would eventually desparsify itself anyways
with disabled discards). Note it in the docs for the user.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Acked-by: Eric Blake <eblake@redhat.com>