Documentation: move driver-api/isapnp to userspace-api/

driver-api/isapnp documents /proc interfaces for interfacing directly
with ISA Plug & Play devices, not any kind of API for kernel developers,
and should thus also live under userspace-api/.

Also fix a few issues while we're at it.

Cc: Jaroslav Kysela <perex@perex.cz>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231221124816.2978000-2-vegard.nossum@oracle.com