audit: document /proc/PID/loginuid

Describe the /proc/PID/loginuid interface in Documentation/ABI/stable that
was added 2005-02-01 by commit 1e2d1492e178 ("[PATCH] audit: handle
loginuid through proc")

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