include: point to the tests in list.c as examples

Even with the documentation, the list.c tests are the best examples.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Chase Douglas <chase.douglas@canonical.com>

include/list.h

@@ -28,6 +28,7 @@
 
 /**
  * @file Classic doubly-link circular list implementation.
+ * For real usage examples of the linked list, see the file test/list.c
  *
  * Example:
  * We need to keep a list of struct foo in the parent struct bar, i.e. what