audit: document /proc/PID/sessionid

Describe the /proc/PID/loginuid interface in Documentation/ABI/stable that
was added 2008-03-13 in commit 1e0bd755 ("[PATCH] export sessionid
alongside the loginuid in procfs")

Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>