feat: add description for policy set

[steph409]

Description

small proposed change: some policy sets, e.g. this one have a description. It would be great if this description is picked up, when rolled out by the module.

small remark: running ./avm pr-check gave me 2 errors for unchanged role_assignment.

Type of Change

• [ ] Non-module change (e.g. CI/CD, documentation, etc.)
• [ ] Azure Verified Module updates:
  • [ ] Bugfix containing backwards compatible bug fixes, and I have NOT bumped the MAJOR or MINOR version in locals.version.tf.json:
    • [ ] Someone has opened a bug report issue, and I have included "Closes #{bug_report_issue_number}" in the PR description.
    • [ ] The bug was found by the module author, and no one has opened an issue to report it yet.
  • [X] Feature update backwards compatible feature updates, and I have bumped the MINOR version in locals.version.tf.json.
  • [ ] Breaking changes and I have bumped the MAJOR version in locals.version.tf.json.
  • [ ] Update to documentation

Checklist

• [X] I'm sure there are no other open Pull Requests for the same update/change
• [ ] My corresponding pipelines / checks run clean and green without any errors or warnings
• [x] I did run all pre-commit checks