documentation: Add atomic_long_t to atomic_ops.txt

maintainers on how to implement atomic counter, bitops, and spinlock

interfaces properly.

	The atomic_t type should be defined as a signed integer.

Also, it should be made opaque such that any kind of cast to a normal

C integer type will fail.  Something like the following should

suffice:

	The atomic_t type should be defined as a signed integer and

the atomic_long_t type as a signed long integer.  Also, they should

be made opaque such that any kind of cast to a normal C integer type

will fail.  Something like the following should suffice:

	typedef struct { int counter; } atomic_t;

	typedef struct { long counter; } atomic_long_t;

Historically, counter has been declared volatile.  This is now discouraged.

See Documentation/volatile-considered-harmful.txt for the complete rationale.

proper implicit or explicit read memory barrier is needed before reading the

value with atomic_read from another thread.

As with all of the atomic_ interfaces, replace the leading "atomic_"

with "atomic_long_" to operate on atomic_long_t.

The second interface can be used at runtime, as in:

	struct foo { atomic_t counter; };