Move question about which network to bridge to en.yml, give it more context

I recently brought up a vagrant VM which uses `public_network` as `config.vm.network`. I was then presented with this question: 

```
==> default: Available bridged network interfaces:
1) en0: Wi-Fi (AirPort)
2) en1: Thunderbolt 1
3) en2: Thunderbolt 2
4) bridge0
5) p2p0
    default: What interface should the network bridge to?
```

I had no idea, honestly. And then I Googled what to select, and the answers were pretty much unanimous:

https://github.com/mitchellh/vagrant/issues/1226#issuecomment-12929493
https://github.com/mitchellh/vagrant/issues/886#issuecomment-45163341
https://www.newmediadenver.com/blog/public-network-vagrant-vm

You select the one which is connected to the internet. I thought a good idea would be to add this piece of knowledge to the question above, so it would now look like this:
```
==> default: Available bridged network interfaces:
1) en0: Wi-Fi (AirPort)
2) en1: Thunderbolt 1
3) en2: Thunderbolt 2
4) bridge0
5) p2p0
    default: What interface should the network bridge to? (Usually the internet-connected interface)
```

Tried to keep it as short as possible.