atomic_ops.txt: mention local_t

local_t is a variant of atomic_t and has related ops to match.
Add reference for local_t documentation to atomic_ops.txt.

Signed-off-by: Grant Grundler <grundler@parisc-linux.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>