Update libpq documentation for PQconndefaults() change.  Add section about thread-safeness of the library.
Update libpq documentation for PQconndefaults() change.  Add section
about thread-safeness of the library.

ashwinstar/gpdb,edespino/gpdb,xuegang/gpdb,lintzc/gpdb,jmcatamney/gpdb,pavanvd/postgres-xl,zaksoup/gpdb,greenplum-db/gpdb,lintzc/gpdb,CraigHarris/gpdb,ahachete/gpdb,techdragon/Postgres-XL,edespino/gpdb,50wu/gpdb,adam8157/gpdb,edespino/gpdb,50wu/gpdb,xuegang/gpdb,chrishajas/gpdb,xuegang/gpdb,ahachete/gpdb,zaksoup/gpdb,xuegang/gpdb,zeroae/postgres-xl,Chibin/gpdb,ashwinstar/gpdb,adam8157/gpdb,arcivanov/postgres-xl,pavanvd/postgres-xl,randomtask1155/gpdb,atris/gpdb,yazun/postgres-xl,yuanzhao/gpdb,0x0FFF/gpdb,atris/gpdb,kaknikhil/gpdb,kmjungersen/PostgresXL,randomtask1155/gpdb,postmind-net/postgres-xl,greenplum-db/gpdb,xuegang/gpdb,lintzc/gpdb,snaga/postgres-xl,pavanvd/postgres-xl,randomtask1155/gpdb,atris/gpdb,greenplum-db/gpdb,Chibin/gpdb,lpetrov-pivotal/gpdb,tpostgres-projects/tPostgres,lpetrov-pivotal/gpdb,tangp3/gpdb,snaga/postgres-xl,tangp3/gpdb,atris/gpdb,rubikloud/gpdb,yazun/postgres-xl,janebeckman/gpdb,foyzur/gpdb,ashwinstar/gpdb,lintzc/gpdb,greenplum-db/gpdb,xuegang/gpdb,CraigHarris/gpdb,chrishajas/gpdb,cjcjameson/gpdb,edespino/gpdb,yuanzhao/gpdb,CraigHarris/gpdb,rubikloud/gpdb,xinzweb/gpdb,Chibin/gpdb,xinzweb/gpdb,Quikling/gpdb,lisakowen/gpdb,lisakowen/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,yazun/postgres-xl,ahachete/gpdb,Chibin/gpdb,50wu/gpdb,rvs/gpdb,Quikling/gpdb,rvs/gpdb,jmcatamney/gpdb,foyzur/gpdb,Quikling/gpdb,jmcatamney/gpdb,atris/gpdb,rubikloud/gpdb,kaknikhil/gpdb,xinzweb/gpdb,arcivanov/postgres-xl,postmind-net/postgres-xl,zaksoup/gpdb,tpostgres-projects/tPostgres,ovr/postgres-xl,ovr/postgres-xl,snaga/postgres-xl,oberstet/postgres-xl,jmcatamney/gpdb,cjcjameson/gpdb,lintzc/gpdb,lintzc/gpdb,CraigHarris/gpdb,Postgres-XL/Postgres-XL,adam8157/gpdb,Postgres-XL/Postgres-XL,yuanzhao/gpdb,arcivanov/postgres-xl,tangp3/gpdb,kaknikhil/gpdb,Postgres-XL/Postgres-XL,cjcjameson/gpdb,arcivanov/postgres-xl,CraigHarris/gpdb,edespino/gpdb,adam8157/gpdb,CraigHarris/gpdb,techdragon/Postgres-XL,ahachete/gpdb,lpetrov-pivotal/gpdb,xinzweb/gpdb,rubikloud/gpdb,zeroae/postgres-xl,foyzur/gpdb,randomtask1155/gpdb,rvs/gpdb,techdragon/Postgres-XL,CraigHarris/gpdb,zeroae/postgres-xl,yuanzhao/gpdb,zaksoup/gpdb,zeroae/postgres-xl,royc1/gpdb,ovr/postgres-xl,foyzur/gpdb,yuanzhao/gpdb,tpostgres-projects/tPostgres,CraigHarris/gpdb,arcivanov/postgres-xl,tpostgres-projects/tPostgres,tangp3/gpdb,cjcjameson/gpdb,janebeckman/gpdb,postmind-net/postgres-xl,pavanvd/postgres-xl,rvs/gpdb,janebeckman/gpdb,cjcjameson/gpdb,royc1/gpdb,royc1/gpdb,kaknikhil/gpdb,0x0FFF/gpdb,rubikloud/gpdb,kmjungersen/PostgresXL,0x0FFF/gpdb,rvs/gpdb,lisakowen/gpdb,ovr/postgres-xl,zaksoup/gpdb,50wu/gpdb,adam8157/gpdb,royc1/gpdb,yuanzhao/gpdb,kmjungersen/PostgresXL,royc1/gpdb,0x0FFF/gpdb,adam8157/gpdb,tpostgres-projects/tPostgres,Quikling/gpdb,ashwinstar/gpdb,Postgres-XL/Postgres-XL,chrishajas/gpdb,zaksoup/gpdb,50wu/gpdb,janebeckman/gpdb,oberstet/postgres-xl,50wu/gpdb,edespino/gpdb,cjcjameson/gpdb,ahachete/gpdb,xinzweb/gpdb,lisakowen/gpdb,Chibin/gpdb,chrishajas/gpdb,edespino/gpdb,50wu/gpdb,zeroae/postgres-xl,foyzur/gpdb,ahachete/gpdb,50wu/gpdb,tangp3/gpdb,zaksoup/gpdb,0x0FFF/gpdb,techdragon/Postgres-XL,zaksoup/gpdb,janebeckman/gpdb,lpetrov-pivotal/gpdb,oberstet/postgres-xl,kaknikhil/gpdb,yuanzhao/gpdb,lisakowen/gpdb,tangp3/gpdb,rvs/gpdb,janebeckman/gpdb,kaknikhil/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,kmjungersen/PostgresXL,lisakowen/gpdb,snaga/postgres-xl,ashwinstar/gpdb,janebeckman/gpdb,xinzweb/gpdb,xuegang/gpdb,edespino/gpdb,xuegang/gpdb,lintzc/gpdb,royc1/gpdb,chrishajas/gpdb,Chibin/gpdb,kaknikhil/gpdb,atris/gpdb,ovr/postgres-xl,oberstet/postgres-xl,Quikling/gpdb,xuegang/gpdb,lintzc/gpdb,ashwinstar/gpdb,0x0FFF/gpdb,jmcatamney/gpdb,oberstet/postgres-xl,lpetrov-pivotal/gpdb,greenplum-db/gpdb,yazun/postgres-xl,randomtask1155/gpdb,yazun/postgres-xl,lpetrov-pivotal/gpdb,yuanzhao/gpdb,pavanvd/postgres-xl,kaknikhil/gpdb,CraigHarris/gpdb,postmind-net/postgres-xl,atris/gpdb,atris/gpdb,rvs/gpdb,tangp3/gpdb,lisakowen/gpdb,Postgres-XL/Postgres-XL,yuanzhao/gpdb,tangp3/gpdb,janebeckman/gpdb,Quikling/gpdb,lintzc/gpdb,jmcatamney/gpdb,cjcjameson/gpdb,foyzur/gpdb,rvs/gpdb,yuanzhao/gpdb,janebeckman/gpdb,randomtask1155/gpdb,rvs/gpdb,0x0FFF/gpdb,xinzweb/gpdb,lisakowen/gpdb,foyzur/gpdb,cjcjameson/gpdb,randomtask1155/gpdb,arcivanov/postgres-xl,Chibin/gpdb,postmind-net/postgres-xl,adam8157/gpdb,kmjungersen/PostgresXL,ahachete/gpdb,ashwinstar/gpdb,cjcjameson/gpdb,snaga/postgres-xl,kaknikhil/gpdb,greenplum-db/gpdb,lpetrov-pivotal/gpdb,ahachete/gpdb,kaknikhil/gpdb,rvs/gpdb,foyzur/gpdb,chrishajas/gpdb,edespino/gpdb,xinzweb/gpdb,0x0FFF/gpdb,cjcjameson/gpdb,royc1/gpdb,techdragon/Postgres-XL,lpetrov-pivotal/gpdb,adam8157/gpdb,Chibin/gpdb,rubikloud/gpdb,rubikloud/gpdb,ashwinstar/gpdb,Chibin/gpdb,royc1/gpdb,randomtask1155/gpdb,Chibin/gpdb,Quikling/gpdb,Quikling/gpdb,rubikloud/gpdb,janebeckman/gpdb,Quikling/gpdb,Quikling/gpdb,edespino/gpdb,chrishajas/gpdb,chrishajas/gpdb