Improve documentation for ol.Feature

This adds a bit more substance to the `ol.Feature` documentation.
This is a tough thing to explain.  What do you think about something like this:

Typically, a feature has a single geometry property.  You can set this with the `setGeometry` method and get it with `getGeometry`.  It is possible to store more than one geometry on a feature.  By default, the geometry used for rendering is set as a property named `'geometry'`.  If you want to use another geometry property for rendering, use the `setGeometryName` method.