tools/memory-model: Explain syntactic and semantic dependencies

Paul Heidekrüger pointed out that the Linux Kernel Memory Model
documentation doesn't mention the distinction between syntactic and
semantic dependencies.  This is an important difference, because the
compiler can easily break dependencies that are only syntactic, not
semantic.

This patch adds a few paragraphs to the LKMM documentation explaining
these issues and illustrating how they can matter.

Suggested-by: Paul Heidekrüger <paul.heidekrueger@in.tum.de>
Reviewed-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>