Added some sentences describing the `grad_fn` argument of functions decorated by `tf.custom_gradient`, because people coming from a math background may think of a function's derivatives as its Jacobian matrix, not understanding how it can be a function `grad_fn` and what the argument `grad_ys` means.
Added some sentences describing the `grad_fn` argument of functions decorated by `tf.custom_gradient`, because people coming from a math background may think of a function's derivatives as its Jacobian matrix, not understanding how it can be a function `grad_fn` and what the argument `grad_ys` means.

PiperOrigin-RevId: 221152230

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