Documentation: devres: Add missing INPUT function

devm_input_allocate_device() got introduced with commit 2be975c6


("Input: introduce managed input devices (add devres support)").
Add this function to the list of managed interfaces within the
devres documentation.

Signed-off-by: Alexander Kurz <akurz@blala.de>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>