docs: Expand on GitHub exclude and repos fields.

Created by: keegancsmith

The documentation for the GitHub external service is a bit sparse for exclude since it doesn't explain what you can put in the exclude field. By including examples (and expanding on the description) in the schema it becomes better documented both in our docs and in the editor.

Test plan: Loaded the docs site in dev and took the above screenshots

Part of #2025 (closed)