docs - dblink limitations (#4572)
docs - dblink limitations (#4572)

* docs - dblink works w/ dbs of same greenplum major version

* cannot dblink to postgresql, remove ref

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