In log_filename documentation, mention that strftime is not used directly to expand the pattern specifier.
In log_filename documentation, mention that strftime is not used directly to
expand the pattern specifier.

Per gripe from Josh Drake.

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