BIP339: clarify fetching

A node may always fetch a transactions using the txid.

bip-0339.mediawiki

@@ -43,7 +43,8 @@ announcing and fetching transactions.
 # The protocol version of nodes implementing this BIP must be set to 70016 or higher.
 # The wtxidrelay message MUST be sent in response to a version message from a peer whose protocol version is >= 70016 and prior to sending a verack. A wtxidrelay message received after a verack message MUST be ignored or treated as invalid.
 # A new inv type MSG_WTX (0x00000005) is added, for use in both inv messages and getdata requests, indicating that the hash being referenced is a transaction's wtxid. In the case of getdata requests, MSG_WTX implies that the transaction being requested should be serialized with witness as well, as described in BIP 144.
-# After a node has sent and received a wtxidrelay message to/from a given peer, the node is required to use the MSG_WTX inv-type when announcing transactions to that peer, or requesting announced transactions from that peer.
+# After a node has received a wtxidrelay message from a peer, the node MUST use the MSG_WTX inv type when announcing transactions to that peer.
+# After a node has received a wtxidrelay message from a peer, the node SHOULD use a MSG_WTX getdata message to request any announced transactions. A node MAY still request transactions from that peer using MSG_TX getdata messages, such as for transactions not recently announced by that peer (like the parents of recently announced transactions).
 
 ==Backward compatibility==