[Doc] Note that v3 onion addresses are not supported
[Doc] Note that v3 onion addresses are not supported

v3 onion addresses are 256 bits, which is currently beyond what our P2P
protocol allows in `addr` messages. Until such time that [BIP155]
(https://github.com/bitcoin/bips/blob/master/bip-0155.mediawiki) can be
implemented, note in the tor.md doc that the `-externalip` configuration
 flag should not be used with v3 onion addresses.

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