Update the comment in nodes.h to cover Cardinality

By chance I discovered that the comment for the typedefs of "double"s
does not cover Cardinality.  Should we update that comment accordingly,
maybe something like below?

- * Typedefs for identifying qualifier selectivities and plan costs as such.
- * These are just plain "double"s, but declaring a variable as Selectivity
- * or Cost makes the intent more obvious.
+ * Typedefs for identifying qualifier selectivities, plan costs and
+ * estimated rows or other count as such.  These are just plain "double"s,
+ * but declaring a variable as Selectivity, Cost or Cardinality makes the
+ * intent more obvious.

Thanks
Richard