Documentation unittests are seriously awesome

[Anonymouse via Digitalmars-d]

On Friday, 23 September 2016 at 20:42:26 UTC, Nick Sabalausky 
wrote:
> Just had to say it. Been using it a lot this week.
>
> Referring to this stuff:
>
> ----------------------------
> /// Does foo stuff
> void foo()
> {}
>
> ///
> unittest
> {
>     // Tests and docs in one, 2-for-1 win!
> }
> ----------------------------
>
> And all just built-in. Freaking sweet.

ELI5 please. What does this do and how is it different from 
normal practice? (I assume having the docs together with the 
tests is what's special here.)