docs($q): add note about optional arguments for promise.then()

- Add note about optional arguments in the promise.then api

Closes #8562
PR (#14754)

Author: wesleycho

src/ng/q.js

@@ -135,7 +135,7 @@
  *
  * **Methods**
  *
- * - `then(successCallback, errorCallback, notifyCallback)` – regardless of when the promise was or
+ * - `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
@@ -146,7 +146,8 @@
  *   with the value which is resolved in that promise using
  *   [promise chaining](http://www.html5rocks.com/en/tutorials/es6/promises/#toc-promises-queues)).
  *   It also notifies via the return value of the `notifyCallback` method. The promise cannot be
- *   resolved or rejected from the notifyCallback method.
+ *   resolved or rejected from the notifyCallback method. The errorCallback and notifyCallback
+ *   arguments are optional.
  *
  * - `catch(errorCallback)` – shorthand for `promise.then(null, errorCallback)`
  *