doc: Update synchronize_rcu() definition in whatisRCU.txt

doc: Update synchronize_rcu() definition in whatisRCU.txt

The synchronize_rcu() definition based on RW-locks in whatisRCU.txt
does not meet the "Memory-Barrier Guarantees" in Requirements.html;
for example, the following SB-like test:

    P0:                      P1:

    WRITE_ONCE(x, 1);        WRITE_ONCE(y, 1);
    synchronize_rcu();       smp_mb();
    r0 = READ_ONCE(y);       r1 = READ_ONCE(x);

should not be allowed to reach the state "r0 = 0 AND r1 = 0", but
the current write_lock()+write_unlock() definition can not ensure
this.  This commit therefore inserts an smp_mb__after_spinlock()
in order to cause this synchronize_rcu() implementation to provide
this memory-barrier guarantee.

Suggested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>