documentation: Fix control dependency and identical stores

The summary of the "CONTROL DEPENDENCIES" section incorrectly states that
barrier() may be used to prevent compiler reordering when more than one
leg of the control-dependent "if" statement start with identical stores.
This is incorrect at high optimization levels.  This commit therefore
updates the summary to match the detailed description.

Reported by: Jianyu Zhan <nasa4836@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>