Adjust autotuning recommendations

This doesn't need normative text.

This adds @ianswett's recommendation about only sending flow control
frames when there are other frames to send.  Though I added another note
about the peer being explicitly blocked, which I think is a useful
transition to the next paragraph.

Closes #2083.

draft-ietf-quic-transport.md

@@ -763,10 +763,12 @@ larger resource commitments at the receiver.  Thus there is a trade-off between
 resource commitment and overhead when determining how large a limit is
 advertised.
 
-A receiver MAY use an autotuning mechanism to tune the frequency and amount of
+A receiver can use an autotuning mechanism to tune the frequency and amount of
 advertised additional credit based on a round-trip time estimate and the rate at
 which the receiving application consumes data, similar to common TCP
-implementations.
+implementations.  Sending frames related to flow control only when there are
+other frames to send or when a peer is blocked ensures that flow control doesn't
+cause extra packets to be sent.
 
 If a sender runs out of flow control credit, it will be unable to send new data
 and is considered blocked.  It is generally considered best to not let the