Extend the Notifications section of the documentation

[bmeike]

The documentation here: https://deploy-preview-3643--realm.netlify.com/docs/java/latest/#notifications

.. could be improved. The distinctions between the two kinds of notification: one with a list of changes and the other with an updated object could be clarified. The current doc separates the description of the latter into two sections, divided by a description of the former. ... and it never describes registering a listener for a RealmResults at all.