docs: Fix incorrect usage description of ctx.save_for_backward

Author: harshaljanjani

beginner_source/examples_autograd/polynomial_custom_function.py

@@ -33,8 +33,10 @@ def forward(ctx, input):
         """
         In the forward pass we receive a Tensor containing the input and return
         a Tensor containing the output. ctx is a context object that can be used
-        to stash information for backward computation. You can cache arbitrary
-        objects for use in the backward pass using the ctx.save_for_backward method.
+        to stash information for backward computation. You can cache tensors for
+        use in the backward pass using the ctx.save_for_backward method. Other
+        objects can be stored directly as attributes on the ctx object, such as 
+        ctx.my_object = my_object.
         """
         ctx.save_for_backward(input)
         return 0.5 * (5 * input ** 3 - 3 * input)