[DOCS] Adds missing breaking change (#20057)

docs/migration/migrate_6_0.asciidoc

@@ -6,6 +6,22 @@ your application to Kibana 6.0.
 
 See also <<release-highlights>> and <<release-notes>>. 
 
+[float]
+=== Saved objects `bulk_get` and `find` endpoints renamed
+//{pull}17512[#17512]
+
+*Details:*
+To support sending larger and more expressive queries to the Saved Objects find
+API, we added a POST endpoint for find and therefore needed a way to disambiguate
+the find API from from the POST API used to create Saved Objects.
+
+*Impact:*
+Starting in {kib} 6.3, endpoints that are not simple CRUD operations on a single
+object will be named and their name will start with an underscore. For example,
+{es} `GET /api/saved_objects` is now `GET /api/saved_objects/_find`,
+`GET /api/saved_objects/{type}` is now `GET /api/saved_objects/_find?type={type}`,
+and `GET /api/saved_objects/bulk_get` is now `GET /api/saved_objects/_bulk_get`.
+
 [float]
 === Types removed from Kibana index
 *Details:*