Document keyword escaping rules

[btiernay]

Clear and concise description of the problem

While this is mentioned in the release notes:

• https://typespec.io/docs/release-notes/release-2023-03-13#allow-escaping-identifiers-using-backticks
  Currently the documentation does not specify the keyword escaping rules such as in this example from the Playground:

Checklist

• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.