Minor code review for json.c.
Minor code review for json.c.

Improve commenting, conform to project style for use of ++ etc.
No functional changes.

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