doc: Remove note on smp_mb during synchronize_rcu

As per this thread [1], it seems this smp_mb() isn't needed anymore:
"So the smp_mb() that I was trying to add doesn't need to be there."

So this commit removes this part from the memory ordering documentation,
and fixes a typo while in the area.

[1] https://lkml.org/lkml/2017/10/6/707

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
1 file changed