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>

Documentation/kref.txt

@@ -128,6 +128,10 @@ since we already have a valid pointer that we own a refcount for.  The
 put needs no lock because nothing tries to get the data without
 already holding a pointer.
 
+In the above example, kref_put() will be called 2 times in both success
+and error paths. This is necessary because the reference count got
+incremented 2 times by kref_init() and kref_get().
+
 Note that the "before" in rule 1 is very important.  You should never
 do something like::