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>

docs/formatsnapshot.rst

@@ -127,9 +127,12 @@ The top-level ``domainsnapshot`` element may contain the following elements:
 
       :since:`Since 8.2.0` the ``snapshot`` attribute supports the ``manual``
       value which instructs the hypervisor to create the snapshot and keep a
-      synchronized state by pausing the VM which allows to snapshot disk
+      synchronized state by pausing the VM (and where supported deactivating
+      the storage backends of the hypervisor), which allows to snapshot disk
       storage from outside of the hypervisor if the storage provider supports
-      it.  The caller is responsible for resuming a VM paused by requesting a
+      it.  VM operations requiring the storage (e.g. blockjobs, migration) should
+      be avoided to ensure that the storage backends can stay deactivated.
+      The caller is responsible for resuming a VM paused by requesting a
       ``manual`` snapshot. When reverting such snapshot, the expectation is that
       the storage is configured in a way where the hypervisor will see the
       correct image state.