tools/memory-model: Add access-marking documentation

This commit adapts the "Concurrency bugs should fear the big bad data-race
detector (part 2)" LWN article (https://lwn.net/Articles/816854/

)
to kernel-documentation form.  This allows more easily updating the
material as needed.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
[ paulmck: Apply Marco Elver feedback. ]
[ paulmck: Update per Akira Yokosawa feedback. ]
Reviewed-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>