Add docs about left-associativity of operators.
Add docs about left-associativity of operators.

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