Documentation: add UUID/GUID to kernel-api

Update kernel-doc notation in lib/uuid.c and then add UUID/GUID
function interfaces to kernel-api.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
[jc: tweaked the uuid_is_valid() kerneldoc]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>