Fix language domain configuration description vs. reality

This sentence does not compute for me: "URL to use for this language, do not any protocol, nor any port.".

Well, we previously suggested they MUST provide a protocol, right? So what about just saying that the language detection is independent on protocol or port, and let them provide it for backwards compatibility / if they want it anyway. The code change should be backwards compatible anyway. BTW why not do this in the same issue as #1250800: Language domain should work regardless of ports or protocols? I don't see them being unrelated.

Tagging, fixing title.

A first rewrite:

The URL to use for this language if URL domains are used for <em>Detection and selection</em>. Leave blank for the default language. <strong>Changing this value may break existing URLs.</strong> Example: Specifying "de.example.com" as language domain for German will result in URLs like "http://de.example.com/contact.

- Removed the 'don'ts': No need to talk about the things that happen automatically anyway.
- Shorter, single example
- Maybe 'Detection and selection' should be a link to that settings page? Not sure.

Well, that part I did not change ;-p
In that case, domain is indeed better.

Still pretty long, but yes.

Looks good to me. Still think this should be rolled into #1250800: Language domain should work regardless of ports or protocols, or if not, marked postponed on that. That issue makes it work as described here, so I think easier would be to get the two in at once.