Update doc mention of diskchecker.pl to add URL for script; retain URL for description.
Update doc mention of diskchecker.pl to add URL for script; retain URL
for description.

Patch to 9.0 and later, where script is mentioned.

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