In pg_upgrade docs, clarify that link mode uses "hard" links.
In pg_upgrade docs, clarify that link mode uses "hard" links.

Backpatch to 9.1 and 9.0.

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