docs: mm: numaperf.rst Add brief description for access class 1.

Try to make minimal changes to the document which already describes
access class 0 in a generic fashion (including IO initiatiors that
are not CPUs).

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>