pg_upgrade docs: mention modification of postgresql.conf in new cluster
pg_upgrade docs: mention modification of postgresql.conf in new cluster

Mention it might be necessary to modify postgresql.conf in the new
cluster to match the old cluster.

Backpatch to 9.2.

Suggested by user.

snaga/postgres-xl,kmjungersen/PostgresXL,snaga/postgres-xl,kmjungersen/PostgresXL,kmjungersen/PostgresXL,arcivanov/postgres-xl,snaga/postgres-xl,postmind-net/postgres-xl,kmjungersen/PostgresXL,postmind-net/postgres-xl,postmind-net/postgres-xl,kmjungersen/PostgresXL,snaga/postgres-xl,postmind-net/postgres-xl,arcivanov/postgres-xl,arcivanov/postgres-xl,postmind-net/postgres-xl,arcivanov/postgres-xl,snaga/postgres-xl,arcivanov/postgres-xl,arcivanov/postgres-xl