Update docs, take 2

docs/developer/advanced/sharing-saved-objects.asciidoc

@@ -4,7 +4,9 @@
 This guide describes the "Sharing saved objects" effort, and the breaking changes that plugin developers need to be aware of for the planned
 8.0 release of {kib}. It also describes how developers can take advantage of this feature.
 
-Whilst new saved object types can still be defined as 'multiple', existing plugins are no longer allowed to switch to shareable saved objects.
+From 8.7.0, as a step towards _zero downtime upgrades_, plugins are no longer allowed to update existing single space saved object types to become shareable.
+Note that new saved object types can still be defined as `'multiple'` or `'multiple-isolated'`.
+
 [[sharing-saved-objects-overview]]
 === Overview