Adding docs about Messages feature

... and some page refactor to organize text flow.

Closes #744. 
please, if possible can you remove the code comments?
It's terrible for UX... mainly on mobile devices. I'd change for:

```jsp
<span class="error">${errors.from('client.name')}</span>
```

It will print "bla bla bla". You can also use the `join` method, just like this:

```jsp
<span class="error">${errors.from('client.name').join(' - ')}</span>
```
In this case the output will be "bla - bla bla". (What do you think?)