Update module-level documentation for preview types

Align the text with the module docs for the stable types, clarifying
the updated current status.

types/preview/index.d.ts

@@ -1,11 +1,21 @@
 /**
-  @module
-  Makes Ember's types for the packages it publishes available by importing this
-  path from `ember-source`:
+  Provides stable type definitions for Ember.js. It is maintained by hand, and
+  the types provided here are unstable and subject to change without warning,
+  though we make a best effort to keep churn to a minimum while we work to
+  stabilize Ember's types.
+
+  To use these type definitions, add these import to any TypeScript file in your
+  Ember app or addon:
 
   ```ts
-  import 'ember-source/preview';
+  import 'ember-source/types';
+  import 'ember-source/types/preview';
   ```
+
+  As Ember's types become more stable, this will automatically give you the
+  latest mix of stable and preview types, with no effort from you.
+
+  @module
  */
 
 // This works because each of these modules presents `declare module` definition