tools: memory-model: Document that the LKMM can easily miss control dependencies

Add a small section to the litmus-tests.txt documentation file for
the Linux Kernel Memory Model explaining that the memory model often
fails to recognize certain control dependencies.

Suggested-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>