docs:  clarify plpython SD and GD dictionary behavior
docs:  clarify plpython SD and GD dictionary behavior

Reported-by: Adam Bielański

Discussion: https://postgr.es/m/153484305538.1370.7605856225879294548@wrigleys.postgresql.org

Backpatch-through: 9.3

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