Clarify request types in cloudfront proxy documentation

[joyceyan]

I just wanted to clarify what types of requests to /js/script.js and /api/event would result in successful responses. I spent a lot of time wondering why a GET request to /api/event would 404 instead of returning a 202. Sending a POST request via curl seems to be fine, and the API docs seem to indicate there isn't a GET request for /api/event so it doesn't seem like it's a problem for that to 404.