[Api-ref] remove "is_public" in snapshot updated description
In api-ref/source/snapshot.inc, when update a snapshot, we can only update
its "display_name" and "display_description", is_public can not be updated,
So this change is to remove "is_public" in update snapshot description.
Closes-bug:
@@ -368,8 +368,6 @@ You can update these attributes:
- ``display_description``, which also changes the ``description`` of
the share snapshot.
-- ``is_public``. Changes the level of visibility.
-
If you try to update other attributes, they retain their previous
values.
