Docs/driver-api/uio-howto: document mmap_prepare callback

Docs/driver-api/uio-howto: document mmap_prepare callback

The UIO howto still documents an mmap callback in struct uio_info.
That field was replaced by mmap_prepare, which takes a struct
vm_area_desc.

A UIO driver following the current howto no longer builds because
struct uio_info has no mmap member. Update the documented callback
signature and matching text to match the current API.

Fixes: 933f05f58ac6 ("uio: replace deprecated mmap hook with mmap_prepare in uio_info")
Signed-off-by: Doehyun Baek <doehyunbaek@gmail.com>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260622181821.1195257-1-doehyunbaek@gmail.com>