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/layer/layer.js

@@ -14,7 +14,6 @@ goog.require('ol.source.Source');
  * @extends {ol.layer.Base}
  * @fires {@link ol.render.Event} ol.render.Event
  * @param {olx.layer.LayerOptions} options Layer options.
- * @todo stability experimental
  * @todo observable brightness {number} the brightness of the layer
  * @todo observable contrast {number} the contrast of the layer
  * @todo observable hue {number} the hue of the layer
@@ -23,6 +22,7 @@ goog.require('ol.source.Source');
  * @todo observable visible {boolean} the visiblity of the layer
  * @todo observable maxResolution {number} the maximum resolution of the layer
  * @todo observable minResolution {number} the minimum resolution of the layer
+ * @todo stability experimental
  * @todo api
  */
 ol.layer.Layer = function(options) {