database-monitoring: keep the module README to usage (#124)

Trim the README to usage; internals are documented privately.

- README keeps what the module deploys, prerequisites, the Cloudsmith
`.terraformrc` block, sizing and cost, upgrade and removal notes,
security-group and egress requirements, and the terraform-docs tables.
- Variable and output descriptions and `main.tf` comments are trimmed to
match.
- Docs and comments only: no resource, input or output changes, and no
version bump.

terraform-docs v0.19.0 + prettier, `terraform fmt -check`, `terraform
validate`, `terraform test` (14 passed), `tflint --recursive` and
`typos` all pass locally.

Co-authored-by: Anshuman Biswas <abiswas@elastio.com>

Author: anchoo2kewl