Up to two full-sized datagrams, not packets

Now that the PTO retransmits Initial and Handshake data, it's possible and likely desirable to bundle multiple QUIC packets in a single datagram.  This change makes it clear that is allowed.

draft-ietf-quic-recovery.md

@@ -514,7 +514,7 @@ and larger thresholds increase loss detection delay.
 
 ## Probe Timeout {#pto}
 
-A Probe Timeout (PTO) triggers sending one or two probe packets when
+A Probe Timeout (PTO) triggers sending one or two probe datagrams when
 ack-eliciting packets are not acknowledged within the expected period of
 time or the handshake has not been completed.  A PTO enables a connection to
 recover from loss of tail packets or acks. The PTO algorithm used in QUIC
@@ -598,8 +598,8 @@ discarded.
 
 When a PTO timer expires, a sender MUST send at least one ack-eliciting packet
 as a probe, unless there is no data available to send.  An endpoint MAY send up
-to two ack-eliciting packets, to avoid an expensive consecutive PTO expiration
-due to a single packet loss.
+to two full sized datagrams containing ack-eliciting packets, to avoid an
+expensive consecutive PTO expiration due to a single lost datagram.
 
 It is possible that the sender has no new or previously-sent data to send.  As
 an example, consider the following sequence of events: new application data is