Rewrite comment for shm_mq_receive_bytes.
Rewrite comment for shm_mq_receive_bytes.

The comment and the code diverged at some point before the initial
commit of this feature, and I failed to notice.

Noted by Tom Lane.

ashwinstar/gpdb,lisakowen/gpdb,lisakowen/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,oberstet/postgres-xl,jmcatamney/gpdb,ovr/postgres-xl,zeroae/postgres-xl,pavanvd/postgres-xl,yazun/postgres-xl,lisakowen/gpdb,yazun/postgres-xl,greenplum-db/gpdb,greenplum-db/gpdb,adam8157/gpdb,ashwinstar/gpdb,lisakowen/gpdb,ovr/postgres-xl,Postgres-XL/Postgres-XL,techdragon/Postgres-XL,yazun/postgres-xl,ovr/postgres-xl,Postgres-XL/Postgres-XL,50wu/gpdb,50wu/gpdb,jmcatamney/gpdb,xinzweb/gpdb,jmcatamney/gpdb,pavanvd/postgres-xl,pavanvd/postgres-xl,yazun/postgres-xl,pavanvd/postgres-xl,oberstet/postgres-xl,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,50wu/gpdb,oberstet/postgres-xl,ashwinstar/gpdb,greenplum-db/gpdb,oberstet/postgres-xl,zeroae/postgres-xl,techdragon/Postgres-XL,xinzweb/gpdb,ashwinstar/gpdb,xinzweb/gpdb,greenplum-db/gpdb,Postgres-XL/Postgres-XL,zeroae/postgres-xl,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,xinzweb/gpdb,50wu/gpdb,greenplum-db/gpdb,adam8157/gpdb,lisakowen/gpdb,xinzweb/gpdb,xinzweb/gpdb,xinzweb/gpdb,adam8157/gpdb,jmcatamney/gpdb,Postgres-XL/Postgres-XL,techdragon/Postgres-XL,lisakowen/gpdb,oberstet/postgres-xl,jmcatamney/gpdb,yazun/postgres-xl,50wu/gpdb,adam8157/gpdb,techdragon/Postgres-XL,adam8157/gpdb,jmcatamney/gpdb,pavanvd/postgres-xl,ashwinstar/gpdb,ovr/postgres-xl,jmcatamney/gpdb,greenplum-db/gpdb,Postgres-XL/Postgres-XL,lisakowen/gpdb,techdragon/Postgres-XL,zeroae/postgres-xl,ovr/postgres-xl,xinzweb/gpdb,zeroae/postgres-xl,50wu/gpdb,50wu/gpdb