docs: devices.rst: better reference documentation docs

There's no need to use either :file: or :doc: tags for documentation,
as automarkup.py automatically converts Documentation/*.rst into
a cross-reference.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/5d9c9949a104d10b537a2d780bccad69a2dc58f9.1623824363.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>