Document workaround for PgBouncer failover errors

What does this MR do?

This documents how to recover from a situation following a Patroni switchover / failover, where PgBouncer fails to RESUME the correct database.

INFO -- : Running: gitlab-ctl pgb-notify --pg-database gitlabhq_production --newhost database7.example.com --user pgbouncer --hostuser gitlab-consul
ERROR -- : STDERR: Error running command: GitlabCtl::Errors::ExecutionError
ERROR -- : STDERR: ERROR: ERROR:  database gitlabhq_production is not paused

Related issues

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