Fix prologue of get_partition_ancestors()

The callers of this function assume that the first Oid in the list
returned by this function corresponds to the immediate parent and the
last on corresponds to the topmost parent.  Make that explicit in the
function prologue.

Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Discussion: https://www.postgresql.org/message-id/CAExHW5vCbATEmht861=G-BFPHNwLUqyeGa_=8-xibJ6Q1UxAeA@mail.gmail.com

src/backend/catalog/partition.c

@@ -123,7 +123,9 @@ get_partition_parent_worker(Relation inhRel, Oid relid, bool *detach_pending)
  * get_partition_ancestors
  *		Obtain ancestors of given relation
  *
- * Returns a list of ancestors of the given relation.
+ * Returns a list of ancestors of the given relation.  The list is ordered:
+ * The first element is the immediate parent and the last one is the topmost
+ * parent in the partition hierarchy.
  *
  * Note: Because this function assumes that the relation whose OID is passed
  * as an argument and each ancestor will have precisely one parent, it should