rc: respond with 202 if prefer-async header is passed

Make rc respond with a 202 status code (instead of 200) if `Prefer: respond-
async` was passed. Keeps backwards compatibility for current clients while also
allowing the OpenAPI schema & generators to differentiate the responses
properly.

docs/content/rc.md

@@ -267,6 +267,13 @@ It is recommended that potentially long running jobs, e.g. `sync/sync`,
 flag to avoid any potential problems with the HTTP request and
 response timing out.
 
+As an alternative to `_async`, you can send the HTTP header
+`Prefer: respond-async` (RFC 7240). This has the same effect as
+`_async = true` but additionally returns HTTP status 202 (Accepted)
+instead of 200, and includes a `Preference-Applied: respond-async`
+response header. The 202 status code makes it easy for clients to distinguish
+an async response from a completed one without inspecting the body.
+
 Starting a job with the `_async` flag:
 
 ```console