Update README.tuplock
Update README.tuplock

Multixact truncation is now handled differently, and this file hadn't
gotten the memo.

Per note from Amit Langote.  I didn't use his patch, though.

Also update the description of infomask bits, which weren't completely up
to date either.  This commit also propagates b01a4f6838 back to 9.3 and
9.4, which apparently I failed to do back then.

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