Greatly simplify and document the usage of JSDoc

This commit simplifies the exports.js plugin so it only relies
on the stability notes to generate the documentation, which
completely decouples it from the exportable API.

As a rule of thumb, whenever something has an 'api' annotation,
it should also have a 'stability' annotation. A more verbose
documentation of ol3 specific annotation usage is available in
the new 'apidoc/readme.md' file.

This commit also modifies all source files to implement these
usage suggestions.

src/ol/map.js

@@ -158,13 +158,13 @@ ol.MapProperty = {
  * @fires {@link ol.MapBrowserEvent} ol.MapBrowserEvent
  * @fires {@link ol.MapEvent} ol.MapEvent
  * @fires {@link ol.render.Event} ol.render.Event
- * @todo stability experimental
- * @todo observable layergroup {ol.layer.LayerGroup} a layer group containing
- *       the layers in this map.
+ * @todo observable layergroup {ol.layer.Group} a layer group containing the
+ *       layers in this map.
  * @todo observable size {ol.Size} the size in pixels of the map in the DOM
  * @todo observable target {string|Element} the Element or id of the Element
  *       that the map is rendered in.
  * @todo observable view {ol.IView} the view that controls this map
+ * @todo stability experimental
  * @todo api
  */
 ol.Map = function(options) {