docs($q): document notify behavior

Closes #3341

src/ng/q.js

@@ -26,6 +26,8 @@
  *       // since this fn executes async in a future turn of the event loop, we need to wrap
  *       // our code into an $apply call so that the model changes are properly observed.
  *       scope.$apply(function() {
+ *         deferred.notify('About to greet ' + name + '.');
+ *
  *         if (okToGreet(name)) {
  *           deferred.resolve('Hello, ' + name + '!');
  *         } else {
@@ -42,6 +44,8 @@
  *     alert('Success: ' + greeting);
  *   }, function(reason) {
  *     alert('Failed: ' + reason);
+ *   }, function(update) {
+ *     alert('Got notification: ' + update);
  *   });
  * </pre>
  *
@@ -60,14 +64,17 @@
  * A new instance of deferred is constructed by calling `$q.defer()`.
  *
  * The purpose of the deferred object is to expose the associated Promise instance as well as APIs
- * that can be used for signaling the successful or unsuccessful completion of the task.
+ * that can be used for signaling the successful or unsuccessful completion, as well as the status
+ * of the task.
  *
  * **Methods**
  *
  * - `resolve(value)` – resolves the derived promise with the `value`. If the value is a rejection
  *   constructed via `$q.reject`, the promise will be rejected instead.
  * - `reject(reason)` – rejects the derived promise with the `reason`. This is equivalent to
  *   resolving it with a rejection constructed via `$q.reject`.
+ * - `notify(value)` - provides updates on the status of the promises execution. This may be called
+ *   multiple times before the promise is either resolved or rejected.
  *
  * **Properties**
  *
@@ -84,12 +91,15 @@
  *
  * **Methods**
  *
- * - `then(successCallback, errorCallback)` – regardless of when the promise was or will be resolved
- *   or rejected, `then` calls one of the success or error callbacks asynchronously as soon as the result
- *   is available. The callbacks are called with a single argument: the result or rejection reason.
+ * - `then(successCallback, errorCallback, notifyCallback)` – regardless of when the promise was or
+ *   will be resolved or rejected, `then` calls one of the success or error callbacks asynchronously
+ *   as soon as the result is available. The callbacks are called with a single argument: the result
+ *   or rejection reason. Additionally, the notify callback may be called zero or more times to
+ *   provide a progress indication, before the promise is resolved or rejected.
  *
  *   This method *returns a new promise* which is resolved or rejected via the return value of the
- *   `successCallback` or `errorCallback`.
+ *   `successCallback`, `errorCallback`. It also notifies via the return value of the `notifyCallback`
+ *   method. The promise can not be resolved or rejected from the notifyCallback method.
  *
  * - `catch(errorCallback)` – shorthand for `promise.then(null, errorCallback)`
  *