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/expression.js

@@ -4,6 +4,16 @@ goog.provide('ol.ExpressionLiteral');
 
 
 /**
+ * Create a new expression. Expressions are used for instance to bind
+ * symbolizer properties to feature attributes.
+ *
+ * Example:
+ *
+ *     // take the color from the color attribute
+ *     color: new ol.Expression('color');
+ *     // take the strokeWidth from the width attribute and multiply by 2.
+ *     strokeWidth: new ol.Expression('width*2');
+ *
  * @constructor
  * @param {string} source Expression to be evaluated.
  */