documentation: Clarify memory-barrier semantics of atomic operations
All value-returning atomic read-modify-write operations must provide full
memory-barrier semantics on both sides of the operation. This commit
clarifies the documentation to make it clear that these memory-barrier
semantics are provided by the operations themselves, not by their callers.
Reported-by: Peter Hurley <peter@hurleysoftware.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>