[RandomAccessIteratorFacade] Also document the elementAt() is a required

method.

[[Imported from SVN: r6155]]

dune/common/iteratorfacades.hh

@@ -438,6 +438,8 @@ namespace Dune
      *
      * // Access the value referred to.
      * Reference dereference() const;
+     * // Access the value at some other location
+     * Reference elementAt(n) const;
      *
      * // Compare for equality with j
      * equals(i);