Fix incorrect IndexOptInfo header comment

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