Add documentation examples for formatValue()

Add example showing how to forward format specifiers.
Add example highlighting the similarity with formattedWrite(), and
comparing their use.
> $(D formatValue)?

Technically not needed, as `formatValue` is the function being documented, so it is already always implicitly `$D`'ed. You can opt *out* by writing `_formatValue` in the documentation body, and it will appear as plain formatValue in the text.

> Also ///

As long as it's a for a function, IMO, there's nothing wrong with a "proper" documentation header. `///` really feels like it should be reserved for trivial getters or, enum values, or whatnot.

That said, I really wish we'd promote `/++ +/` more than `/** */`. Unless you plan to backport to `C`, then `+` is always at least as good as `*`. End rent.