Improve documentation for ol.Feature

This adds a bit more substance to the `ol.Feature` documentation.
I know this is the theory behind how this works, but is it actually possible with the current architecture to use more than one geometry simultaneously - to draw a polygon and label its center point (where the center point is provided by a second geometry)?  I haven't seen anything that leads me to think this is possible right now.  It might be nice if a layer or style could be configured with a geometry property name.