Add some weasel wording about threaded usage of PGresults.
Add some weasel wording about threaded usage of PGresults.

PGresults used to be read-only from the application's viewpoint, but now
that we've exposed various functions that allow modification of a PGresult,
that sweeping statement is no longer accurate.  Noted by Dmitriy Igrishin.

jmcatamney/gpdb,yazun/postgres-xl,tpostgres-projects/tPostgres,oberstet/postgres-xl,techdragon/Postgres-XL,lisakowen/gpdb,xinzweb/gpdb,snaga/postgres-xl,xinzweb/gpdb,lisakowen/gpdb,lisakowen/gpdb,Postgres-XL/Postgres-XL,adam8157/gpdb,kmjungersen/PostgresXL,ashwinstar/gpdb,pavanvd/postgres-xl,ashwinstar/gpdb,postmind-net/postgres-xl,50wu/gpdb,tpostgres-projects/tPostgres,pavanvd/postgres-xl,ovr/postgres-xl,Postgres-XL/Postgres-XL,adam8157/gpdb,zeroae/postgres-xl,50wu/gpdb,techdragon/Postgres-XL,xinzweb/gpdb,ovr/postgres-xl,yazun/postgres-xl,50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,50wu/gpdb,xinzweb/gpdb,xinzweb/gpdb,zeroae/postgres-xl,jmcatamney/gpdb,arcivanov/postgres-xl,50wu/gpdb,postmind-net/postgres-xl,ashwinstar/gpdb,kmjungersen/PostgresXL,tpostgres-projects/tPostgres,greenplum-db/gpdb,adam8157/gpdb,adam8157/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,arcivanov/postgres-xl,yazun/postgres-xl,adam8157/gpdb,yazun/postgres-xl,ashwinstar/gpdb,jmcatamney/gpdb,arcivanov/postgres-xl,arcivanov/postgres-xl,oberstet/postgres-xl,oberstet/postgres-xl,lisakowen/gpdb,snaga/postgres-xl,greenplum-db/gpdb,ovr/postgres-xl,tpostgres-projects/tPostgres,snaga/postgres-xl,Postgres-XL/Postgres-XL,50wu/gpdb,postmind-net/postgres-xl,ashwinstar/gpdb,xinzweb/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,adam8157/gpdb,ovr/postgres-xl,jmcatamney/gpdb,pavanvd/postgres-xl,Postgres-XL/Postgres-XL,greenplum-db/gpdb,snaga/postgres-xl,arcivanov/postgres-xl,pavanvd/postgres-xl,yazun/postgres-xl,lisakowen/gpdb,ashwinstar/gpdb,zeroae/postgres-xl,xinzweb/gpdb,adam8157/gpdb,zeroae/postgres-xl,kmjungersen/PostgresXL,techdragon/Postgres-XL,kmjungersen/PostgresXL,greenplum-db/gpdb,techdragon/Postgres-XL,oberstet/postgres-xl,postmind-net/postgres-xl,Postgres-XL/Postgres-XL,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,zeroae/postgres-xl,pavanvd/postgres-xl,tpostgres-projects/tPostgres,50wu/gpdb,adam8157/gpdb,50wu/gpdb,snaga/postgres-xl,oberstet/postgres-xl,postmind-net/postgres-xl,techdragon/Postgres-XL,jmcatamney/gpdb,ovr/postgres-xl,arcivanov/postgres-xl,kmjungersen/PostgresXL,jmcatamney/gpdb,greenplum-db/gpdb