rtio: MPSC doc comment cleanup

The doc comment relating to mpsc atomics was worded poorly. Remove
the poorly worded doc comment related to atomics and caches.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>

include/zephyr/rtio/rtio_mpsc.h

@@ -33,8 +33,7 @@ extern "C" {
  * by the compiler generated op codes is enough.
  *
  * On SMP atomics *must* be used to ensure the pointers
- * are updated in the correct order and the values are
- * updated core caches correctly.
+ * are updated in the correct order.
  */
 #if defined(CONFIG_SMP)