srcu: Update synchronize_srcu()'s comments

The core of SRCU is changed, but synchronize_srcu()'s comments describe
the old algorithm.  This commit therefore updates them to match the
new algorithm.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>