Add comment on trailing return type with decltype that is used for detecting existence of setUseFixedOrder member function

opm/simulators/linalg/parallelamgbackend.hh

@@ -255,6 +255,7 @@ protected:
     }
 #endif
 
+    // trailing return type with decltype used for detecting existence of setUseFixedOrder member function by overloading the setUseFixedOrder function
     template <typename C>
     auto setUseFixedOrder(C criterion, bool booleanValue) -> decltype(criterion.setUseFixedOrder(booleanValue))
     {