doc: Preallocation does not require writing zeroes

When preallocating an encrypted qcow2 image, it just lets the protocol
driver write data and then does not mark the clusters as zero.
Therefore, reading this image will yield effectively random data.

As such, we have not fulfilled the promise of always writing zeroes when
preallocating an image in a while.  It seems that nobody has really
cared, so change the documentation to conform to qemu's actual behavior.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190711132935.13070-1-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>