docs: kref: Clarify the use of two kref_put() in example code

Eventhough the current documentation explains that the reference count
gets incremented by both kref_init() and kref_get(), it is often
misunderstood that only one instance of kref_put() is needed in the
example code. So let's clarify that a bit.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>