docs: snapshot: Add a note that blockjobs ought to be avoided with 'manual' snapshots

docs: snapshot: Add a note that blockjobs ought to be avoided with 'manual' snapshots

Using a blockjob will reactivate the block nodes in qemu and thus e.g.
qcow2 metadata such as bitmaps may become marked as dirty. Users of
'manual' snapshots ought to avoid those.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>