[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.

doc/tor.md

@@ -46,14 +46,16 @@ config file): *Needed for Tor version 0.2.7.0 and older versions of Tor only. Fo
 versions of Tor see [Section 3](#3-automatically-listen-on-tor).*
 
 	HiddenServiceDir /var/lib/tor/pivx-service/
+	HiddenServiceVersion 2
 	HiddenServicePort 51472 127.0.0.1:51472
 	HiddenServicePort 61472 127.0.0.1:61472
 
 The directory can be different of course, but (both) port numbers should be equal to
 your pivxd's P2P listen port (51472 by default).
 
 	-externalip=X   You can tell pivx about its publicly reachable address using
-	                this option, and this can be a .onion address. Given the above
+	                this option, and this can be a v2 .onion address (v3 .onion
+	                addresses are not supported by the PIVX network). Given the above
 	                configuration, you can find your .onion address in
 	                /var/lib/tor/pivx-service/hostname. For connections
 	                coming from unroutable addresses (such as 127.0.0.1, where the