Comment on default prevout.link.

Author: evoskuil

include/bitcoin/system/chain/prevout.hpp

@@ -54,6 +54,7 @@ class BC_API prevout final
         uint32_t median_time_past{ max_uint32 };
 
         /// node: set to the database record of the input association.
+        /// node: it is necessary that Link::terminal derives from max_uint32.
         uint32_t link;
     };