doc: Merged/updated old docs for Element.classNames, mostly by marking it deprecated.

src/dom/dom.js

@@ -780,11 +780,15 @@ Element.Methods = {
     return Element.getDimensions(element).width;
   },
 
-  /**
+  /** deprecated
    *  Element.classNames(@element) -> [String...]
    *
    *  Returns a new instance of [[Element.ClassNames]], an [[Enumerable]]
    *  object used to read and write CSS class names of `element`.
+   *
+   *  **Deprecated**, please see [[Element.addClassName]],
+   *  [[Element.removeClassName]], and [[Element.hasClassName]]. If you want
+   *  an array of classnames, you can use `$w(element.className)`.
   **/
   classNames: function(element) {
     return new Element.ClassNames(element);