Improve documentation on aws secret manager setup on terraform

[kruskall]

Followup to #389

Is your feature request related to a problem? Please describe.
documentation for setting up the aws secret manager was quite hard to follow

Describe the solution you'd like
Make it clear/consistent with the smoke test we recently added

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.