doc: mention dependency on collation libraries
doc: mention dependency on collation libraries

Document that index storage is dependent on the operating system's
collation library ordering, and any change in that ordering can create
invalid indexes.

Discussion: 20160617154311.GB19359@momjian.us

Backpatch-through: 9.1

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