Add more comments to tqual.c for visibility functions.
Add more comments to tqual.c for visibility functions.

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