Fix incorrect comment for get_agg_clause_costs

Adjust the header comment in get_agg_clause_costs so that it matches what
the function currently does.  No recursive searching has been done ever
since 0a2bc5d61.  It also does not determine the aggtranstype like the
comment claimed. That's all done in preprocess_aggref().
preprocess_aggref also now determines the numOrderedAggs, so remove the
mention that get_agg_clause_costs also calculates "counts".

Normally, since this is just an adjustment of a comment it might not be
worth back-patching, but since this code is new to PG14 and that version
is still in beta, then it seems worth having the comments match.

Discussion: https://postgr.es/m/CAApHDvrrGrTJFPELrjx0CnDtz9B7Jy2XYW3Z2BKifAWLSaJYwQ@mail.gmail.com
Backpatch-though: 14

src/backend/optimizer/prep/prepagg.c

@@ -515,28 +515,24 @@ GetAggInitVal(Datum textInitVal, Oid transtype)
 
 /*
  * get_agg_clause_costs
- *	  Recursively find the Aggref nodes in an expression tree, and
- *	  accumulate cost information about them.
+ *	  Process the PlannerInfo's 'aggtransinfos' and 'agginfos' lists
+ *	  accumulating the cost information about them.
  *
  * 'aggsplit' tells us the expected partial-aggregation mode, which affects
  * the cost estimates.
  *
- * NOTE that the counts/costs are ADDED to those already in *costs ... so
- * the caller is responsible for zeroing the struct initially.
+ * NOTE that the costs are ADDED to those already in *costs ... so the caller
+ * is responsible for zeroing the struct initially.
  *
- * We count the nodes, estimate their execution costs, and estimate the total
- * space needed for their transition state values if all are evaluated in
- * parallel (as would be done in a HashAgg plan).  Also, we check whether
- * partial aggregation is feasible.  See AggClauseCosts for the exact set
- * of statistics collected.
+ * For each AggTransInfo, we add the cost of an aggregate transition using
+ * either the transfn or combinefn depending on the 'aggsplit' value.  We also
+ * account for the costs of any aggfilters and any serializations and
+ * deserializations of the transition state and also estimate the total space
+ * needed for the transition states as if each aggregate's state was stored in
+ * memory concurrently (as would be done in a HashAgg plan).
  *
- * In addition, we mark Aggref nodes with the correct aggtranstype, so
- * that that doesn't need to be done repeatedly.  (That makes this function's
- * name a bit of a misnomer.)
- *
- * This does not descend into subqueries, and so should be used only after
- * reduction of sublinks to subplans, or in contexts where it's known there
- * are no subqueries.  There mustn't be outer-aggregate references either.
+ * For each AggInfo in the 'agginfos' list we add the cost of running the
+ * final function and the direct args, if any.
  */
 void
 get_agg_clause_costs(PlannerInfo *root, AggSplit aggsplit, AggClauseCosts *costs)