Added documentation for function xmlagg.
Added documentation for function xmlagg.

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