message: doc: mention that we enforce port=0 in I2P
diff: @@ -70,3 +70,18 @@ RPC.
  Bitcoin Core uses the [SAM v3.1](https://geti2p.net/en/docs/api/samv3) protocol
  to connect to the I2P network. Any I2P router that supports it can be used.
  +
  +## Ports in I2P and Bitcoin Core
  +
  +Bitcoin Core uses the [SAM v3.1](https://geti2p.net/en/docs/api/samv3)
  +protocol. One particularity of SAM v3.1 is that it does not support ports,
  +unlike newer versions of SAM (v3.2 and up) that do support them and default the
  +port numbers to 0. From the point of view of peers that use newer versions of
  +SAM or other protocols that support ports, a SAM v3.1 peer is connecting to them
  +on port 0, from source port 0.
  +
  +To allow future upgrades to newer versions of SAM, Bitcoin Core sets its
  +listening port to 0 when listening for incoming I2P connections and advertises
  +its own I2P address with port 0. Furthermore, it will not attempt to connect to
  +I2P addresses with a non-zero port number because with SAM v3.1 the destination
  +port (`TO_PORT`) is always set to 0 and is not in the control of Bitcoin Core.
  