Extend keys and screens documentation

Add chapters `Lazy functions` and `Special keys` to the `keys.rst` and `Third-party bars` to `screens.rst`.
On Thu, Sep 18, 2014 at 01:59:17PM -0700, Jakub Kadlčík wrote:
> > @@ -39,3 +39,17 @@ Bars support background colors and gradients, e.g. :code:`bar.Bar(...,
> >  background="#000000")` will give you a black back ground (the default), while
> >  :code:`bar.Bar(..., background=["#000000", "#FFFFFF"])` will give you a
> >  background that fades from black to white.
> > +
> > +
> > +Third-party bars
> > +----------------
> > +
> > +There might be some reasons to use third-party bars. For instance you can come from another window manager and you have already configured dzen2, xmobar, or something else. They can definitely be used with Qtile too.
> > +
> > +::
> > +
> > +        Screen(
> 
> We both had right. It didn't work for me when I have tried it, but it work as you said. The problem is, that I forgot the `-dock` parameter for dzen2.
> 
> So, what about the Third-party bars? Just remove the whole section or rewrite it in the way, that Qtile's bar can do awesome things and there shouldn't be a need for third-party bar, but when it comes to, Qtile can handle them?

If you want to keep it, I would just say something about how third
party bars are supported in Qtile, and they should work out of the box
with no additional configuration on the Qtile side. I don't think we
should discourage people from using 3rd party bars -- people should
use whatever suits their needs best.

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> https://github.com/qtile/qtile/pull/512/files#r17754329