Restore docs "rcu: Restore barrier() to rcu_read_lock() and rcu_read_unlock()"

This restores docs back in ReST format.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
[ paulmck: Added Joel's SoB per Stephen Rothwell feedback. ]
[ paulmck: Joel approved via private email. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>