Update overlooked comment for Gather Merge.

Commit 355d3993 probably should have
done this, but nobody noticed that it was needed.

src/backend/optimizer/plan/createplan.c

@@ -5390,9 +5390,9 @@ make_sort(Plan *lefttree, int numCols,
  * prepare_sort_from_pathkeys
  *	  Prepare to sort according to given pathkeys
  *
- * This is used to set up for both Sort and MergeAppend nodes.  It calculates
- * the executor's representation of the sort key information, and adjusts the
- * plan targetlist if needed to add resjunk sort columns.
+ * This is used to set up for Sort, MergeAppend, and Gather Merge nodes.  It
+ * calculates the executor's representation of the sort key information, and
+ * adjusts the plan targetlist if needed to add resjunk sort columns.
  *
  * Input parameters:
  *	  'lefttree' is the plan node which yields input tuples
@@ -5416,7 +5416,7 @@ make_sort(Plan *lefttree, int numCols,
  *
  * If the pathkeys include expressions that aren't simple Vars, we will
  * usually need to add resjunk items to the input plan's targetlist to
- * compute these expressions, since the Sort/MergeAppend node itself won't
+ * compute these expressions, since a Sort or MergeAppend node itself won't
  * do any such calculations.  If the input plan type isn't one that can do
  * projections, this means adding a Result node just to do the projection.
  * However, the caller can pass adjust_tlist_in_place = TRUE to force the