Docs: explain contrib/pg_stat_statements' handling of GC failure.
Docs: explain contrib/pg_stat_statements' handling of GC failure.

Failure to perform garbage collection now has a user-visible effect, so
explain that and explain that reducing pgss_max is the way to prevent it.
Per gripe from Andrew Dunstan.

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