Explain max_prepared_transactions requirement in isolation tests' README.
Explain max_prepared_transactions requirement in isolation tests' README.

Now that we have a test that requires nondefault settings to pass, it seems
like we'd better mention that detail in the directions about how to run the
tests.

Also do some very minor copy-editing.

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