Add @todo observable documentation for observable properties

This PR adds documentation for observable properties, which will then be pulled into the docs correctly once #1180 is merged.  This is a first pass based on searching for definition of observable properties being defined as enums after lines ending with `Property = {`.  If there are observable properties implemented that don't follow this pattern then they are not included.

I've added simple descriptions based on what I know or could easily figure out, there may be some properties (like preload) that are not correctly described.

I've also added `readonly` annotations where I knew that a property was readonly.  I may have missed some readonly properties.

ol.layer.Base has a bunch of properties but I don't think it is exported so the documentation of these properties will not show up, so I added the documentation to ol.layer.Layer instead even though this isn't really where it should be documented.

src/ol/overlay.js

@@ -57,6 +57,12 @@ ol.OverlayPositioning = {
  * @extends {ol.Object}
  * @param {ol.OverlayOptions} options Overlay options.
  * @todo stability experimental
+ * @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
  */
 ol.Overlay = function(options) {