Remove INCLUDE attributes section from docs.
Remove INCLUDE attributes section from docs.

Discussing covering indexes in a chapter that is mostly about the
behavior of B-Tree operator classes is unnecessary.  The CREATE INDEX
documentation's handling of covering indexes seems sufficient.

Discussion: 4559a4d942b24c081a17ebc7114e9a5ab40758a4@mail.gmail.com

greenplum-db/gpdb,50wu/gpdb,xinzweb/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb,adam8157/gpdb,greenplum-db/gpdb,adam8157/gpdb,greenplum-db/gpdb,adam8157/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb,xinzweb/gpdb,lisakowen/gpdb,50wu/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,xinzweb/gpdb,lisakowen/gpdb,adam8157/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb