clarified spec

dlib/dnn/core_abstract.h

@@ -379,7 +379,8 @@ namespace dlib
         );
         /*!
             requires
-                - forward(x) was called to forward propagate x though the network.
+                - forward(x) was called to forward propagate x though the network.  Moreover,
+                  this was the most recent call to forward().
                 - get_gradient_input() has been set equal to the gradient of this network's
                   output with respect to some loss function.
                 - This instance of solvers has only ever been used with this network.  That