clarified spec

dlib/dnn/core_abstract.h

@@ -916,6 +916,10 @@ namespace dlib
             - Checks if l correctly implements the EXAMPLE_LAYER_ interface defined in
               layers_abstract.h.  Importantly, it computes numerical approximations to the
               gradients and compares them to the outputs of the layer.  
+            - Note that this function is only capable of checking layers that take
+              arbitrary subnetworks as input.  So if you have designed a layer that expects
+              only a certain restricted type of subnetwork then you might get an error when
+              you call this function.
     !*/
 
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