Documentation: atomic_ops: use {READ,WRITE}_ONCE()

While the {READ,WRITE}_ONCE() macros should be used in preference to
ACCESS_ONCE(), the atomic documentation uses the latter exclusively.

To point people in the right direction, and as a step towards the
eventual removal of ACCESS_ONCE(), update the documentation to use the
{READ,WRITE}_ONCE() macros as appropriate.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>