JMenu: Codestyle

- updated comments in docblock
- `$menu->getDefault()` now returns null in case of failure like other menu methods. This shouldn't affect anything, as return is evaluated using `is_object` in places noted below or not at all.
  - [JPatchwaySite](https://github.com/piotr-cz/joomla-cms/blob/master/libraries/cms/pathway/site.php#L40)
  - [JRouterSite](https://github.com/piotr-cz/joomla-cms/blob/master/libraries/cms/router/site.php#L155)

Issue tracker Item: [32222](http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=32222&start=0)