kcsan: Add pointer to access-marking.txt to data_race() bullet

This commit references tools/memory-model/Documentation/access-marking.txt
in the bullet introducing data_race().  The access-marking.txt file
gives advice on when data_race() should and should not be used.

Suggested-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>