Fix docs to not claim ECPG's SET CONNECTION is not thread-aware.
Fix docs to not claim ECPG's SET CONNECTION is not thread-aware.

Changed by: Tsunakawa, Takayuki <tsunakawa.takay@jp.fujitsu.com>

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