Fix incorrect IndexOptInfo header comment

The comment incorrectly indicated that indexcollations[] stored
collations for both key columns and INCLUDE columns, but in reality it
only has elements for the key columns.  canreturn[] didn't get a mention,
so add that while we're here.

Author: Junwang Zhao <zhjwpku@gmail.com>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/CAEG8a3LwbZgMKOQ9CmZarX5DEipKivdHp5PZMOO-riL0w%3DL%3D4A%40mail.gmail.com
Backpatch-through: 14

src/include/nodes/pathnodes.h

@@ -796,9 +796,10 @@ typedef struct RelOptInfo
  * IndexOptInfo
  *		Per-index information for planning/optimization
  *
- *		indexkeys[], indexcollations[] each have ncolumns entries.
- *		opfamily[], and opcintype[]	each have nkeycolumns entries. They do
- *		not contain any information about included attributes.
+ *		indexkeys[] and canreturn[] each have ncolumns entries.
+ *
+ *		indexcollations[], opfamily[], and opcintype[] each have nkeycolumns
+ *		entries.  These don't contain any information about INCLUDE columns.
  *
  *		sortopfamily[], reverse_sort[], and nulls_first[] have
  *		nkeycolumns entries, if the index is ordered; but if it is unordered,