AWS RDS: Add note to clarify the expected value of rds_postgres_instance_type variable

What does this MR do?

Adding some notes to clarify expected values for the rds_postgres_instance_type Terraform variable associated with RDS cluster

Related issues

Closes https://gitlab.com/gitlab-org/quality/gitlab-environment-toolkit/-/issues/237

Author's checklist

When ready for review, the Author applies the workflowready for review label:

• Merge request:
  • Merge Request Title and Description are up to date, accurate, and descriptive
  • MR targeting the appropriate branch
  • MR has a green pipeline
• Code:
  • Check the area changed works as expected. Consider testing it in different environment sizes (1k,3k,10k,etc.).
  • Documentation created/updated in the same MR.
  • If this MR adds an optional configuration - check that all permutations continue to work.
  • For Terraform changes: setup a previous version environment, then run a terraform plan with your new changes and ensure nothing will be destroyed. If anything will be destroyed and this can't be avoided please add a comment to the current MR.
• Create any follow-up issue(s) to support the new feature across other supported cloud providers or advanced configurations. Create 1 issue for each provider/configuration. Contact the Quality Enablement team if unsure.