docs: driver-model: device: Add DEVICE_ATTR_{RO,RW} examples

bus.rst, driver.rst, and hwmon-kernel-api.rst refer to the
DEVICE_ATTR_* macros for devices, but device.rst does not mention them.

Add a paragraph about these helper macros, and use them in the examples.
Retain the old description, as it is still useful for less common values
of mode. Change the names of the example "show" and "store" methods, to
match the expectations of the DEVICE_ATTR_* macros.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20210303133845.3939403-2-geert+renesas@glider.be
Signed-off-by: Jonathan Corbet <corbet@lwn.net>