Clean up handling of dropped columns in NAMEDTUPLESTORE RTEs.

The NAMEDTUPLESTORE patch piggybacked on the infrastructure for
TABLEFUNC/VALUES/CTE RTEs, none of which can ever have dropped columns,
so the possibility was ignored most places.  Fix that, including adding a
specification to parsenodes.h about what it's supposed to look like.

In passing, clean up assorted comments that hadn't been maintained
properly by said patch.

Per bug #14799 from Philippe Beaudoin.  Back-patch to v10.

Discussion: https://postgr.es/m/20170906120005.25630.84360@wrigleys.postgresql.org

src/include/nodes/parsenodes.h

@@ -1025,6 +1025,11 @@ typedef struct RangeTblEntry
 	 * from the catalogs if 'relid' was supplied, but we'd still need these
 	 * for TupleDesc-based ENRs, so we might as well always store the type
 	 * info here).
+	 *
+	 * For ENRs only, we have to consider the possibility of dropped columns.
+	 * A dropped column is included in these lists, but it will have zeroes in
+	 * all three lists (as well as an empty-string entry in eref).  Testing
+	 * for zero coltype is the standard way to detect a dropped column.
 	 */
 	List	   *coltypes;		/* OID list of column type OIDs */
 	List	   *coltypmods;		/* integer list of column typmods */