remove Tile.Image:insertBackBuffer and add API docs for Tile.Image:cloneMarkup

lib/OpenLayers/Tile/Image.js

@@ -320,21 +320,13 @@ OpenLayers.Tile.Image = OpenLayers.Class(OpenLayers.Tile, {
     },
 
     /**
-     * Method: insertBackBuffer
-     * Create a copy of this tile's markup and insert it to the layer
-     * div.
+     * Method: cloneMarkup
+     * Clone this tile's markup.
      *
      * Returns:
-     * {DOMElement} The back buffer.
+     * {DOMElement} The markup, or undefined if the tile has no image
+     * or if it's currently loading.
      */
-    insertBackBuffer: function() {
-        var frame = this.frame.cloneNode(false);
-        this.layer.div.insertBefore(frame, this.frame);
-        frame.appendChild(this.imgDiv);
-        this.imgDiv = null;
-        return frame;
-    },
-
     cloneMarkup: function() {
         if(!this.imgDiv || this.isLoading) {
             return;