Update dev docs

* Add Sharing Saved Objects section in new dev docs; this intentionally
links to the legacy docs page (which is intended to be viewed by both
internal and external developers)
* Remove docs examples of `namespaceType: 'single'` since we want folks
to stop using that.

dev_docs/key_concepts/saved_objects.mdx

@@ -72,3 +72,17 @@ Sometimes Saved Objects end up persisted inside another Saved Object. We call th
  issues with edits propagating - since an entity can only exist in a single place.
   Note that from the end user stand point, we don’t use these terms “by reference” and “by value”.
 
+## Sharing Saved Objects
+
+Starting in Kibana 7.12, saved objects can be shared to multiple spaces. The "space behavior" is determined for each object type depending
+on how it is registered.
+
+If you are adding a **new** object type, when you register it:
+
+1. Use `namespaceType: 'multiple-isolated'` to make these objects exist in exactly one space
+2. Use `namespaceType: 'multiple'` to make these objects exist in one *or more* spaces
+3. Use `namespaceType: 'agnostic'` if you want these objects to always exist in all spaces
+
+If you have an **existing** "legacy" object type that is not shareable (using `namespaceType: 'single'`), see the [legacy developer guide
+for Sharing Saved Objects](https://www.elastic.co/guide/en/kibana/master/sharing-saved-objects.html) for details on steps you need to take
+to make sure this is converted to `namespaceType: 'multiple-isolated'` or `namespaceType: 'multiple'` in the 8.0 release.

dev_docs/tutorials/saved_objects.mdx

@@ -19,7 +19,7 @@ import { SavedObjectsType } from 'src/core/server';
 export const dashboardVisualization: SavedObjectsType = {
   name: 'dashboard_visualization', [1]
   hidden: false,
-  namespaceType: 'single',
+  namespaceType: 'multiple-isolated', [2]
   mappings: {
     dynamic: false,
     properties: {
@@ -41,6 +41,10 @@ export const dashboardVisualization: SavedObjectsType = {
 [1] Since the name of a Saved Object type forms part of the url path for the public Saved Objects HTTP API, 
 these should follow our API URL path convention and always be written as snake case.
 
+[2] This field determines "space behavior" -- whether these objects can exist in one space, multiple spaces, or all spaces. This value means
+that objects of this type can only exist in a single space. See
+<DocLink id="kibDevDocsSavedObjectsIntro" section="sharing-saved-objects" text="Sharing Saved Objects"/> for more information.
+
 **src/plugins/my_plugin/server/saved_objects/index.ts**
 
 ```ts

docs/developer/architecture/core/saved-objects-service.asciidoc

@@ -45,7 +45,7 @@ import { SavedObjectsType } from 'src/core/server';
 export const dashboardVisualization: SavedObjectsType = {
   name: 'dashboard_visualization', // <1>
   hidden: false,
-  namespaceType: 'single',
+  namespaceType: 'multiple-isolated', // <2>
   mappings: {
     dynamic: false,
     properties: {
@@ -66,6 +66,8 @@ export const dashboardVisualization: SavedObjectsType = {
 <1> Since the name of a Saved Object type forms part of the url path for the
 public Saved Objects HTTP API, these should follow our API URL path convention
 and always be written as snake case.
+<2> This field determines "space behavior" -- whether these objects can exist in one space, multiple spaces, or all spaces. This value means
+that objects of this type can only exist in a single space. See <<sharing-saved-objects,Sharing Saved Objects>> for more information.
 
 .src/plugins/my_plugin/server/saved_objects/index.ts
 [source,typescript]