Updates to Db module docs

1. Make dump a required config option, as the module does not actually
have a default dump location and doesn't really do anything if its not
provided.

2. The documentation states that setting "cleanup" option to true,
reloads the dump "after" each test. In my mind this would mean after a
test is run, the database is restored to its original states.
In actual fact the code actually restores the dump "before" each test.
This means that after a test is run, the database is in has the test
changes. This difference is important if one is having more tests
run (maybe a different suite) after the current suite which might not
be using the Db module.