doc: Describe better handling of indexes in ALTER TABLE ATTACH PARTITION

doc: Describe better handling of indexes in ALTER TABLE ATTACH PARTITION

When ALTER TABLE ... ATTACH PARTITION matches partition indexes to the
parent table's indexes, invalid indexes are skipped.  This commit
improves the documentation to describe what e90e9275f56 has changed:
invalid indexes are skipped, and only valid indexes are considered for a
match.

Author: Mohamed Ali <moali.pg@gmail.com>
Reviewed-by: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/CAGnOmWpAMaE-BOkpwM6mJnHcpS2QZ8yLSSaqmz+vryEsbCWWWA@mail.gmail.com
Backpatch-through: 14