input: Documentation: corrections for notifier.rst

Add 'ledstate' to the keyboard_notifier_param struct info and tell
which header file contains that struct.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: linux-input@vger.kernel.org
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/r/20210302223523.20130-7-rdunlap@infradead.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Input: create a book with Linux Input documentation

Now that all files under Documentation/input follows the ReST markup
language, rename them to *.rst and create a book for the Linux Input
subsystem.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>