[docs] illegal code samples

[Julien Palard]

Hi Mario,

> It would be helpful, perhaps, to color the box of the illegal code another way, or, in the interest of eventual printout, a double-frame box, an "ILLEGAL" text block, or some other visual cue.

I like the idea, would you mind opening an issue on bugs.python.org to keep track of it? And if you feel trying to implement it, don't hesitate to open a PR :)

-- 
Julien Palard
https://mdk.fr