Clarify `listenonion`

> This new feature is enabled by default if Bitcoin Core is listening, and a connection to Tor can be made. It can be configured with the -listenonion, -torcontrol and -torpassword settings. To show verbose debugging information, pass -debug=tor.

But it is correct to say that the feature is enabled *regardless* of whether a connection to Tor can be made.

I propose to clarify that so that users can eliminate these in their logs (when `listen=1` and no Tor).
And I think it's okay to clarify about the `listen` option, because on several occasions when I read this before I always assumed `listening` meant `server=1` which cost me a lot of time in troubleshooting.

```
2016-10-24 06:19:22.551029 tor: Error connecting to Tor control socket
2016-10-24 06:19:22.551700 tor: Not connected to Tor control port 127.0.0.1:9051, trying to reconnect
```

### What version of bitcoin-core are you using?
0.12.1

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