Input: document that unregistering managed devices is not necessary

Apparently some users of managed input devices are confused whether
input_unregister_device() is needed when working with them. Clarify
this in the kernel doc for devm_input_allocate_device(): in most cases
there is no need to call neither input_unregister_device() nor
input_free_device() when working with managed devices.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>