move transitionEffect to Layer.Grid.prototype, and discourage the use of transitionEffect:"resize" on non-opaque layers

lib/OpenLayers/Layer/Grid.js

@@ -91,6 +91,22 @@ OpenLayers.Layer.Grid = OpenLayers.Class(OpenLayers.Layer.HTTPRequest, {
      */
     buffer: 0,
 
+    /**
+     * APIProperty: transitionEffect
+     * {String} The transition effect to use when the map is zoomed.
+     *
+     * Two posible values:
+     *  - *null* No transition effect (the default).
+     *  - *resize*  Existing tiles are resized on zoom to provide a visual
+     *    effect of the zoom having taken place immediately.  As the
+     *    new tiles become available, they are drawn over top of the
+     *    resized tiles.
+     *
+     * Using "resize" on non-opaque layers can cause undesired visual
+     * effects. This is therefore discouraged.
+     */
+    transitionEffect: null,
+
     /**
      * APIProperty: numLoadingTiles
      * {Integer} How many tiles are still loading?