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: NGrant Grundler <grundler@parisc-linux.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/atomic_ops.txt

@@ -14,6 +14,10 @@ suffice:
 
 	typedef struct { volatile int counter; } atomic_t;
 
+local_t is very similar to atomic_t. If the counter is per CPU and only
+updated by one CPU, local_t is probably more appropriate. Please see
+Documentation/local_ops.txt for the semantics of local_t.
+
 	The first operations to implement for atomic_t's are the
 initializers and plain reads.