docs: Include uaccess docs from the right file

Documentation/core-api/kernel-api.rst was including kerneldoc comments from
arch/x86/include/asm/uaccess_32.h, but the relevant comments moved to
.../uaccess.h some time ago.  Correct the include to pick up the comments
and eliminate a warning.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>