Update comment for Cardinality typedef

Author: Richard Guo <guofenglinux@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CAMbWs4-Zd7Yy80RL1NdskLLo-wz6QoqsbC5TKs%3D3yZxG3BT_aA%40mail.gmail.com

src/include/nodes/nodes.h

@@ -230,9 +230,9 @@ extern bool equal(const void *a, const void *b);
 
 
 /*
- * Typedefs for identifying qualifier selectivities and plan costs as such.
+ * Typedefs for identifying qualifier selectivities, plan costs, and row
- * These are just plain "double"s, but declaring a variable as Selectivity
+ * counts as such.  These are just plain "double"s, but declaring a variable
- * or Cost makes the intent more obvious.
+ * as Selectivity, Cost, or Cardinality makes the intent more obvious.
  *
  * These could have gone into plannodes.h or some such, but many files
  * depend on them...