Add comments about the diversity of signal functionality.  Came from Makefile.global.
Add comments about the diversity of signal functionality.  Came from
Makefile.global.

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