docs(orderBy): clarify behavior of default comparator wrt `null`

Document how `orderBy`'s default comparator handles `null` values.

Fixes #15293

Closes #15304

docs/content/guide/migration.ngdoc

@@ -1125,7 +1125,11 @@ and "$dependentProvider" would have actually accomplished something and changed
 app. This is no longer possible within a single module.
 
 
+### Filters (`orderBy`)
 
+- due to [a097aa95](https://github.com/angular/angular.js/commit/a097aa95b7c78beab6d1b7d521c25f7d9d7843d9),
+  `orderBy` now treats `null` values (which in JavaScript have type `object`) as having a string
+  represenation of `'null'`.
 
 
 ### Animation (`ngAnimate`)
@@ -1229,7 +1233,6 @@ or simply use:
 
 
 
-
 ## Migrating from 1.0 to 1.2
 
 

src/ng/filter/orderBy.js

@@ -86,6 +86,9 @@
  *
  * **Note:** If you notice numbers not being sorted as expected, make sure they are actually being
  *           saved as numbers and not strings.
+ * **Note:** For the purpose of sorting, `null` values are treated as the string `'null'` (i.e.
+ *           `type: 'string'`, `value: 'null'`). This may cause unexpected sort order relative to
+ *           other values.
  *
  * @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