feat: Adds description field to CfnResourcePolicyProps & removes note for private preview

[maastha]

Proposed changes

Jira ticket: CLOUDP-311360
CLOUDP-295769

Adds description field to CfnResourcePolicyProps & removes note for private preview.

Type of change:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as
  expected)
• This change requires a documentation update

Required Checklist:

• I have signed the MongoDB CLA
• I have added tests that prove my fix is effective or that my feature works
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code
• I have tested the CDK constructor in a CFN stack. See TESTING.md
• If changes include removal or addition of 3rd party GitHub actions, I updated our internal document. Reach out to the APIx Integration slack channel to get access to the internal document.

Further comments

[AgustinBettati]

LGTM