Add documentation examples for formatValue()

Add example showing how to forward format specifiers.
Add example highlighting the similarity with formattedWrite(), and
comparing their use.
> Is it? I think it's printed in green, but not in monotype. But the DDoc blocks lower had other D symbols too.

I'm actually unsure. I know Andrei has made some "useless" comments when function names were being `$D`'ed in documentation. But no, I don't actually know what it does. IMO, there's nothing wrong with tagging with `$D` anyways. I just thought I'd mention it. Anybody know what happens *exactly* to the names of documented functions in DDoc?