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

Documentation/userspace-api/index.rst

@@ -31,6 +31,7 @@ place where this information is gathered.
    vduse
    futex2
    tee
+   isapnp
 
 .. only::  subproject and html
 

Documentation/userspace-api/isapnp.rst

@@ -0,0 +1,15 @@
+=======================
+ISA Plug & Play support
+=======================
+
+Interface /proc/isapnp
+======================
+
+The interface was removed in kernel 2.5.53. See pnp.rst for more details.
+
+Interface /proc/bus/isapnp
+==========================
+
+This directory allows access to ISA PnP cards and logical devices.
+The regular files contain the contents of ISA PnP registers for
+a logical device.