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