Add migration section for deprecated update where

modules/ROOT/pages/migration/index.adoc

@@ -765,3 +765,43 @@ type Movie @node {
   title: String! @private
 }
 ----
+
+
+=== Deprecated `where` field in `update` input
+
+The `where` field for nested update operations has been deprecated to be moved within the `update` input field.
+The `where` in its deprecated location is a no-op for all nested operations apart from `update`.
+
+For example, the following mutation is using the deprecated syntax:
+
+```graphql
+mutation {
+    updateUsers(
+        where: { name: { eq: "Darrell" } }
+        update: {
+            posts: {
+                where: { node: { title: { eq: "Version 6 Release Notes" } } }
+                update: { node: { title: { set: "Version 6 Release Announcement" } } }
+            }
+        }
+    )
+}
+```
+
+It should be modified to move the `where` inside the `update` operation:
+
+```graphql
+mutation {
+    updateUsers(
+        where: { name: { eq: "Darrell" } }
+        update: {
+            posts: {
+                update: {
+                    where: { node: { title: { eq: "Version 6 Release Notes" } } }
+                    node: { title: { set: "Version 6 Release Announcement" } }
+                }
+            }
+        }
+    )
+}
+```