Fix documentation about joining pg_locks to other views.

The advice to join to pg_prepared_xacts via the transaction column was not
updated when the transaction column was replaced by virtualtransaction.
Since it's not quite obvious how to do that join, give an explicit example.
For consistency also give an example for the adjacent case of joining to
pg_stat_activity.  And link-ify the view references too, just because we
can.  Per bug #9840 from Alexey Bashtanov.

Michael Paquier and Tom Lane