Add comment to ABILITY_FEEDING macro

 There should be some explanation of what was done in 2ba7ad5 otherwise that's pretty sneaky.

data/core/macros/abilities.cfg

@@ -269,6 +269,7 @@ Enemy units cannot see this unit while it is in deep water, except if they have
 #define ABILITY_FEEDING
     # Canned definition of the Feeding ability to be included in an
     # [abilities] clause.
+    # This works because of data/lua/feeding.lua
     [dummy]
         id=feeding
         name= _ "feeding"