Fix outdated convert_saop_to_hashed_saop comment

In 29f45e2, we added support for optimizing the execution of NOT
IN(values) by using a hash table instead of a linear search over the
array.  That commit neglected to update the header comment for
convert_saop_to_hashed_saop() to mention this fact.  Here we fix that.

Author: James Coleman
Discussion: https://postgr.es/m/CAAaqYe99NUpAPcxgchGstgM23fmiGjqQPot8627YgkBgNt=BfA@mail.gmail.com
Backpatch-through: 15, where 29f45e2 was added.

Author: david-rowley

src/backend/optimizer/util/clauses.c

@@ -2117,7 +2117,8 @@ eval_const_expressions(PlannerInfo *root, Node *node)
  *
  * We'll use a hash table if all of the following conditions are met:
  * 1. The 2nd argument of the array contain only Consts.
- * 2. useOr is true.
+ * 2. useOr is true or there is a valid negator operator for the
+ *	  ScalarArrayOpExpr's opno.
  * 3. There's valid hash function for both left and righthand operands and
  *	  these hash functions are the same.
  * 4. If the array contains enough elements for us to consider it to be