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/overlay.js

@@ -56,13 +56,13 @@ ol.OverlayPositioning = {
  * @constructor
  * @extends {ol.Object}
  * @param {olx.OverlayOptions} options Overlay options.
- * @todo stability stable
  * @todo observable element {Element} the Element containing the overlay
  * @todo observable map {ol.Map} the map that the overlay is part of
  * @todo observable position {ol.Coordinate} the spatial point that the overlay
  *       is anchored at
  * @todo observable positioning {ol.OverlayPositioning} how the overlay is
  *       positioned relative to its point on the map
+ * @todo stability stable
  * @todo api
  */
 ol.Overlay = function(options) {