Fix misleading comment for get_cheapest_group_keys_order

The header comment for get_cheapest_group_keys_order() claimed that the
output arguments were set to a newly allocated list which may be freed by
the calling function, however, this was not always true as the function
would simply leave these arguments untouched in some cases.

This tripped me up when working on 1349d2790 as I mistakenly assumed I
could perform a list_concat with the output parameters.  That turned out
bad due to list_concat modifying the original input lists.

In passing, make it more clear that the number of distinct values is
important to reduce tiebreaks during sorts.  Also, explain what the
n_preordered parameter means.

Backpatch-through: 15, where get_cheapest_group_keys_order was introduced.

src/backend/optimizer/path/pathkeys.c

@@ -587,22 +587,25 @@ pathkey_sort_cost_comparator(const void *_a, const void *_b)
 
 /*
  * get_cheapest_group_keys_order
- *		Reorders the group pathkeys/clauses to minimize the comparison cost.
+ *		Reorders the group pathkeys / clauses to minimize the comparison cost.
  *
- * Given a list of pathkeys, we try to reorder them in a way that minimizes
- * the CPU cost of sorting. This depends mainly on the cost of comparator
- * function (the pathkeys may use different data types) and the number of
- * distinct values in each column (which affects the number of comparator
- * calls for the following pathkeys).
+ * Given the list of pathkeys in '*group_pathkeys', we try to arrange these
+ * in an order that minimizes the sort costs that will be incurred by the
+ * GROUP BY.  The costs mainly depend on the cost of the sort comparator
+ * function(s) and the number of distinct values in each column of the GROUP
+ * BY clause (*group_clauses).  Sorting on subsequent columns is only required
+ * for tiebreak situations where two values sort equally.
  *
  * In case the input is partially sorted, only the remaining pathkeys are
- * considered.
+ * considered.  'n_preordered' denotes how many of the leading *group_pathkeys
+ * the input is presorted by.
  *
- * Returns true if the keys/clauses have been reordered (or might have been),
- * and a new list is returned through an argument. The list is a new copy
- * and may be freed using list_free.
- *
- * Returns false if no reordering was possible.
+ * Returns true and sets *group_pathkeys and *group_clauses to the newly
+ * ordered versions of the lists that were passed in via these parameters.
+ * If no reordering was deemed necessary then we return false, in which case
+ * the *group_pathkeys and *group_clauses lists are left untouched. The
+ * original *group_pathkeys and *group_clauses parameter values are never
+ * destructively modified in place.
  */
 static bool
 get_cheapest_group_keys_order(PlannerInfo *root, double nrows,