MEDIUM: config: inform the user about the deprecatedness of "block" rules

It's just a warning emitted once.
diff --git a/include/types/global.h b/include/types/global.h
index 21433e8..967d3b4 100644
--- a/include/types/global.h
+++ b/include/types/global.h
@@ -173,7 +173,7 @@
 extern unsigned int warned;     /* bitfield of a few warnings to emit just once */
 
 /* bit values to go with "warned" above */
-/* #define WARN_* */
+#define WARN_BLOCK_DEPRECATED       0x00000001
 
 /* to be used with warned and WARN_* */
 static inline int already_warned(unsigned int warning)