Add more content for the API doc

Involves additions to (there will be subsequent pull requests for other
classes):
- ol.Attribution
- ol.Collection
- ol.Expression
- ol.Feature
- ol.Geolocation
- ol.Map
- ol.Object
- ol.Overlay
- ol.Projection
- ol.View2D
- ol.control.Attribution
- ol.control.Control
- ol.control.FullScreen
- ol.control.Logo
- ol.control.MousePosition
- ol.control.ScaleLine
- ol.control.Zoom
- ol.control.ZoomSlider
- ol.dom.Input
- ol.filter.Filter
- ol.filter.Geometry
- ol.filter.Logical

src/ol/control/zoomcontrol.js

@@ -20,6 +20,9 @@ ol.control.ZOOM_DURATION = 250;
 
 
 /**
+ * Create a new control with 2 buttons, one for zoom in and one for zoom out.
+ * This control is part of the default controls of a map. To style this control
+ * use css selectors .ol-zoom-in and .ol-zoom-out.
  * @constructor
  * @extends {ol.control.Control}
  * @param {ol.control.ZoomOptions=} opt_options Zoom options.