doc: Add refcount analogy to What is RCU

The reader-writer-lock analogy is a useful way to think about RCU, but
it is not always applicable.  It is useful to have other analogies to
work with, and particularly to emphasise that no single analogy is
perfect.

This patch add a "RCU as reference count" to the "what is RCU" document.

See https://lwn.net/Articles/872559/



[ paulmck: Apply Akira Yokosawa feedback. ]
Reviewed-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>