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>

Documentation/dev-tools/kcsan.rst

@@ -106,7 +106,9 @@ the below options are available:
 
 * KCSAN understands the ``data_race(expr)`` annotation, which tells KCSAN that
   any data races due to accesses in ``expr`` should be ignored and resulting
-  behaviour when encountering a data race is deemed safe.
+  behaviour when encountering a data race is deemed safe.  Please see
+  ``tools/memory-model/Documentation/access-marking.txt`` in the kernel source
+  tree for more information.
 
 * Disabling data race detection for entire functions can be accomplished by
   using the function attribute ``__no_kcsan``::