Improve documentation on aws secret manager setup on terraform

Followup to https://github.com/elastic/apm-aws-lambda/issues/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.