atomic_ops.txt: mention local_t

	typedef struct { volatile int counter; } atomic_t;

	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

	The first operations to implement for atomic_t's are the

initializers and plain reads.

initializers and plain reads.