docs(orderBy): Clarify behavior of default comparator

docs/content/guide/migration.ngdoc

@@ -1226,6 +1226,12 @@ or simply use:
   As communicated before, IE8 is no longer supported.
 
 
+### Filters (`orderBy`)
+
+- due to [a097aa95](https://github.com/angular/angular.js/commit/a097aa95b7c78beab6d1b7d521c25f7d9d7843d9),
+  the default orderBy comparator now treats null values as the string 'null'
+
+
 
 
 

src/ng/filter/orderBy.js

@@ -86,6 +86,8 @@
  *
  * **Note:** If you notice numbers not being sorted as expected, make sure they are actually being
  *           saved as numbers and not strings.
+ * **Note:** The above algorithm converts `null` values (normally of type `object`) to the string
+ *           `'null'` (so has type `string`). This may cause unexpected sort order.
  *
  * @param {Array|ArrayLike} collection - The collection (array or array-like object) to sort.
  * @param {(Function|string|Array.<Function|string>)=} expression - A predicate (or list of