chore: add schema docs to AWS::Serverless::Api

[hoffa]

Issue #, if available

#1133

Description of changes

Description of how you validated changes

Checklist

• Adheres to the development guidelines
• Add/update unit tests using:
  • Correct values
  • Bad/wrong values (None, empty, wrong type, length, etc.)
  • Intrinsic Functions
• Add/update integration tests
• Update documentation
• Verify transformed template deploys and application functions as expected
• Do these changes include any template validations?
  • Did the newly validated properties support intrinsics prior to adding the validations? (If unsure, please review Intrinsic Functions before proceeding).
    • Does the pull request ensure that intrinsics remain functional with the new validations?

Examples?

Please reach out in the comments, if you want to add an example. Examples will be
added to sam init through https://github.com/awslabs/aws-sam-cli-app-templates/

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.