docs(migration): in 1.3, global controllers are disabled by default

Closes #10775

Author: Narretz

Diff for: docs/content/guide/migration.ngdoc

@@ -15,6 +15,44 @@ which drives many of these changes.
 
 # Migrating from 1.2 to 1.3
 
+## Controllers
+
+Due to [3f2232b5](https://github.com/angular/angular.js/commit/3f2232b5a181512fac23775b1df4a6ebda67d018),
+`$controller` will no longer look for controllers on `window`.
+The old behavior of looking on `window` for controllers was originally intended
+for use in examples, demos, and toy apps. We found that allowing global controller
+functions encouraged poor practices, so we resolved to disable this behavior by
+default.
+
+To migrate, register your controllers with modules rather than exposing them
+as globals:
+
+Before:
+
+```javascript
+function MyController() {
+  // ...
+}
+```
+
+After:
+
+```javascript
+angular.module('myApp', []).controller('MyController', [function() {
+  // ...
+}]);
+```
+
+Although it's not recommended, you can re-enable the old behavior like this:
+
+```javascript
+angular.module('myModule').config(['$controllerProvider', function($controllerProvider) {
+  // this option might be handy for migrating old apps, but please don't use it
+  // in new ones!
+  $controllerProvider.allowGlobals();
+}]);
+```
+
 ## Angular Expression Parsing (`$parse` + `$interpolate`)
 
 - due to [77ada4c8](https://github.com/angular/angular.js/commit/77ada4c82d6b8fc6d977c26f3cdb48c2f5fbe5a5),