fix incorrect docs about what gradient is computed

dlib/cuda/tensor_tools.h

@@ -1639,18 +1639,19 @@ namespace dlib { namespace tt
 
     void gelu_gradient (
         tensor& grad,
-        const tensor& dest,
+        const tensor& src,
         const tensor& gradient_input
     );
     /*!
         requires
-            - have_same_dimensions(dest,gradient_input) == true
-            - have_same_dimensions(dest,grad) == true
+            - have_same_dimensions(src,gradient_input) == true
+            - have_same_dimensions(src,grad) == true
         ensures
-            - This function computes the gradient of f() with respect to SRC and stores
-              it to grad.  Moreover, if is_same_object(grad,gradient_input)==true then
-              the output is assigned to grad, replacing its previous contents.
-              Otherwise the output is added to grad.
+            - Recalling that dest is the output of gelu(dest,src), let f(src) ==
+              dot(gradient_input,dest). Then this function computes the gradient of f() with respect
+              to src and stores it to grad.  Moreover, if is_same_object(grad,gradient_input)==true
+              then the output is assigned to grad, replacing its previous contents.  Otherwise the
+              output is added to grad.
             - This function supports in-place operation, i.e. having
               is_same_object(grad, gradient_input)==true
     !*/