While the {READ,WRITE}_ONCE() macros should be used in preference to
ACCESS_ONCE(), the circular buffer 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
READ_ONCE(), as ACCESS_ONCE() is only used in a reader context in the
circular buffer documentation.
Signed-off-by: NMark Rutland <mark.rutland@arm.com>
Acked-by: NDavid Howells <dhowells@redhat.com>
Acked-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>