Add test for sort_order and order_by date in notifications component.

See #5452

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8153 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Improve inline docs for BP_Notifications_Notification::get()

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8154 cdf35c40-ae34-48e0-9cc9-0c9da1808c22