Documentation: userspace-api: correct spelling

Correct spelling problems for Documentation/userspace-api/ as reported
by codespell.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Cc: Sean Young <sean@mess.org>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: linux-media@vger.kernel.org
Link: https://lore.kernel.org/r/20230129231053.20863-10-rdunlap@infradead.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

iommufd: Document overview of iommufd

Add iommufd into the documentation tree, and supply initial documentation.
Much of this is linked from code comments by kdoc.

Link: https://lore.kernel.org/r/5-v6-a196d26f289e+11787-iommufd_jgg@nvidia.com
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>