[discretefunction.default] clearly doc the need for manual ctors

dune/gdt/discretefunction/default.hh

@@ -164,13 +164,14 @@ public:
   {
   }
 
+  // manual copy ctor needed bc. of the storage provider
   DiscreteFunction(const ThisType& other)
     : VectorProviderBaseType(new VectorType(other.vector()))
     , BaseType(other.space(), VectorProviderBaseType::storage_access(), other.name())
   {
   }
 
-  //! \todo is this intentionally c&p of the copy ctor?
+  // manual move ctor needed bc. of the storage provider
   DiscreteFunction(ThisType&& source)
     : VectorProviderBaseType(new VectorType(source.vector()))
     , BaseType(source.space(), VectorProviderBaseType::storage_access(), source.name())