docs: hooks: Document when shutoff-reason argument was introduced

Introduced in v10.5.0-rc1~52, qemu and lxc hook scripts are
executed with additional argument: shutoff reason. But wording of
our docs make it looks like it's been that way forever. Make it
clear this is `recent` feature.

Resolves: https://gitlab.com/libvirt/libvirt/-/issues/766
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>

docs/hooks.rst

@@ -211,7 +211,10 @@ operation. There is no specific operation to indicate a "restart" is occurring.
       /etc/libvirt/hooks/qemu guest_name stopped end -
 
    Then, after libvirt has released all resources, the hook is called again,
-   :since:`since 0.9.0`, to allow any additional resource cleanup:
+   :since:`since 0.9.0`, to allow any additional resource cleanup.
+   The ``shutoff-reason`` argument (:since:`since 10.5.0`; before that
+   '-' was passed instead) provides the reason for the shutdown of
+   the domain.
 
    ::
 
@@ -331,7 +334,10 @@ operation. There is no specific operation to indicate a "restart" is occurring.
       /etc/libvirt/hooks/lxc guest_name stopped end -
 
    Then, after libvirt has released all resources, the hook is called again,
-   :since:`since 0.9.0`, to allow any additional resource cleanup:
+   :since:`since 0.9.0`, to allow any additional resource cleanup.
+   The ``shutoff-reason`` argument (:since:`since 10.5.0`; before that
+   '-' was passed instead) provides the reason for the shutdown of
+   the domain.
 
    ::