Improve the "You should enable Extended ORPort" log message.

As can be seen in https://lists.torproject.org/pipermail/tor-relays/2014-February/003908.html people get confused about the new "You should enable Extended ORPort" log message.

This is kind of related to legacy/trac#10957 (moved) , but in this ticket we should figure out a better log message for the short term.

We currently log:

We are a bridge with a pluggable transport
proxy but the Extended ORPort is disabled. The Extended ORPort helps Tor
communicate with the pluggable transport proxy. Please enable it using
the ExtORPort torrc option.

we should at least mention that users can simply put ExtORPort auto in their torrc to fix this issue. And maybe we should include an online link to further documentation about ExtORPort.

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