Clarify what the client should receiving on email resending

It was a little unclear what the client should do when asked to send a validation email and provides a `send_attempt` value that is not greater than previous attempts. As this is intended to be for when a client mistakenly sends the request twice, it makes logical sense that the implication was to simply resend a success value so the client doesn't error even when an email may have been sent on the first attempt.

This behaviour was also mimicked in Synapse/Sydent.

api/identity/definitions/request_email_validation.yaml

@@ -39,7 +39,8 @@ properties:
       avoid repeatedly sending the same email in the case of request
       retries between the POSTing user and the identity server.
       The client should increment this value if they desire a new
-      email (e.g. a reminder) to be sent.
+      email (e.g. a reminder) to be sent. If they do not, the server
+      should return a success but not resend the email.
     example: 1
   next_link:
     type: string