pgsql: Fix documentation about joining pg_locks to other views.

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

Branch
------
REL9_0_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/0de9963b0b4e692e6c712a7bb1ee0faf61afa9f0

Modified Files
--------------
doc/src/sgml/catalogs.sgml |   25 ++++++++++++++++++-------
1 file changed, 18 insertions(+), 7 deletions(-)