rcu: Document that rcu_barrier() hurries lazy callbacks

This commit adds to the rcu_barrier() kerneldoc header stating that this
function hurries lazy callbacks and that it does not normally result in
additional RCU grace periods.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree.c

@@ -3800,6 +3800,11 @@ static void rcu_barrier_handler(void *cpu_in)
  * to complete.  For example, if there are no RCU callbacks queued anywhere
  * in the system, then rcu_barrier() is within its rights to return
  * immediately, without waiting for anything, much less an RCU grace period.
+ * In fact, rcu_barrier() will normally not result in any RCU grace periods
+ * beyond those that were already destined to be executed.
+ *
+ * In kernels built with CONFIG_RCU_LAZY=y, this function also hurries all
+ * pending lazy RCU callbacks.
  */
 void rcu_barrier(void)
 {