pata_platform: Document `pio_mask' module parameter

Add MODULE_PARM_DESC documentation and a kernel-parameters.txt entry.

Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2103212023190.21463@angie.orcam.me.uk


Signed-off-by: Jens Axboe <axboe@kernel.dk>