[doc] Mark new criterion mechanism as experimental

dune/solvers/solvers/criterion.hh

@@ -10,6 +10,11 @@
 
 #include <dune/common/stringutility.hh>
 
+
+/**
+ * \attention Everything contained in this headere is experimental and may change in the near future.
+ */
+
 namespace Dune {
 
   namespace Solvers {

dune/solvers/solvers/loopsolver.hh

@@ -63,6 +63,12 @@ public:
      * This method forward all arguments to the constructor of
      * Criterion. So you can pass a Criterion or any argument
      * list supported by the Criterion constructors.
+     *
+     * Notice, that the old hard-wired criteria are still active.
+     * If you don't supply any criteria here, the you will get
+     * exactly the old behaviour.
+     *
+     * \attention This is an experimental feature that may still change in the near future.
      */
     template<class... Args>
     void addCriterion(Args&&... args)