docs - updated foreign data wrapper intro. (#8554)
docs - updated foreign data wrapper intro. (#8554)

--Removed references to foreign data wrappers install with GPDB
--Modified note. Added information stating PostgrSQL FDWs only access master.
50wu/gpdb,jmcatamney/gpdb,lisakowen/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb,xinzweb/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,adam8157/gpdb,jmcatamney/gpdb,xinzweb/gpdb,greenplum-db/gpdb,adam8157/gpdb,jmcatamney/gpdb,xinzweb/gpdb,greenplum-db/gpdb,adam8157/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,50wu/gpdb,xinzweb/gpdb,adam8157/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,ashwinstar/gpdb,xinzweb/gpdb,ashwinstar/gpdb,lisakowen/gpdb,ashwinstar/gpdb,50wu/gpdb,ashwinstar/gpdb,lisakowen/gpdb,lisakowen/gpdb,adam8157/gpdb,ashwinstar/gpdb,adam8157/gpdb,50wu/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,xinzweb/gpdb,adam8157/gpdb,ashwinstar/gpdb,50wu/gpdb,50wu/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb,greenplum-db/gpdb,xinzweb/gpdb,jmcatamney/gpdb