charms: Update (qa)staging DB user management process

We've switched to new (qa)staging databases that are entirely controlled
by developers.  The procedure still needs some tidying up, but this
should work for now.

explanation/charms.rst

@@ -211,8 +211,8 @@ to provide charms with database credentials rather than relating them to
 complications when the database is deployed in a different region from some
 of our applications).  As a result, we need to do some manual user
 management in these environments.  On staging and qastaging, developers can
-do most of this themselves when adding new charms to those existing
-deployment environments.
+do this themselves when adding new charms to those existing deployment
+environments.
 
 Taking the librarian as an example: ``charm/launchpad-librarian/layer.yaml``
 lists the ``binaryfile-expire``, ``librarian``, ``librarianfeedswift``, and
@@ -244,24 +244,25 @@ like this, again replacing ``<secret>`` with the generated password:
 .. code-block:: yaml
 
     launchpad_qastaging_librarian:
-      master: "postgresql://juju_launchpad-librarian:<secret>@pamola.internal:6432/launchpad_qastaging?connect_timeout=10"
+      master: "postgresql://juju_launchpad-librarian:<secret>@database-ps5-1.qastaging.lp.internal:6432/launchpad_qastaging?connect_timeout=10"
       standbys: []
 
 In the connection string URL, the database host, port, and name (in this
-case, ``pamola.internal``, ``6432``, and ``launchpad_qastaging``
-respectively) should match those of other entries in ``db_connections``.
+case, ``database-ps5-1.qastaging.lp.internal``, ``6432``, and
+``launchpad_qastaging`` respectively) should match those of other entries in
+``db_connections``.
 
 The configuration for the ``pgbouncer`` connection pooler must also be
-updated to match, which currently requires help from IS.  On
-``pamola.internal``, IS should take the relevant username/password pair from
-the ``deploy-secrets`` file above and add it to
-``/etc/pgbouncer/userlist.txt``.
+updated to match.  For now, take the relevant username/password pair from
+the ``deploy-secrets`` file above; then, on each of the ``postgresql`` units
+in ``[email protected]``, add this pair to
+``/etc/pgbouncer/userlist.txt`` and run ``sudo systemctl reload
+pgbouncer.service``.  In the near future this will be turned into a Mojo
+spec.
 
 Staging works similarly with the obvious substitutions of ``staging`` for
-``qastaging``.  The qastaging and staging environments currently share a
-``pgbouncer``; as a result, while the user still has to be created on both
-database clusters, the passwords for a given user on qastaging and staging
-must be identical.
+``qastaging``, and using
+``[email protected]``.
 
 Production works similarly, except that IS needs to generate the user on the
 production database, add it to the production ``pgbouncer`` by editing