specify that Retry-After header should be used to manage polling of the file upload session status

Author: ewdurbin

peps/pep-0694.rst

@@ -672,6 +672,8 @@ The successful response includes the following:
       }
     }
 
+A `Retry-After` response header **MUST** be present
+to indicate to clients when they should next poll for an updated status.
 
 Besides the ``meta`` key, which has the same format as the request JSON, the success response has
 the following keys:
@@ -731,8 +733,14 @@ request), then it may return a ``202 Accepted`` response
 and set the status of the File Upload Session to ``processing``.
 
 In either case, the server should include a ``Location`` header pointing back to the File Upload
-Session status URL, and if the server returned a ``202 Accepted``, the client may poll that URL to
-watch for the status to change.
+Session status URL.
+
+Servers **MUST** allow clients to poll the File Upload Session status URL
+to watch for the status to change.
+If the server responds with a ``202 Accepted``,
+clients may poll the File Upload Session status URL to watch for the status to change.
+Clients **SHOULD** respect the `Retry-After` header value
+of the File Upload Session status response.
 
 If an error occurs, the appropriate ``4xx`` code should be returned, as described in the
 :ref:`session-errors` section.