[Doc] Specify the return type of adapted functions

I'm looking at `flip` right now, but this also applies to basically all the rest of the library. The documentation says

> **Synopsis**
>
> ``` c++
> template
> flip_adaptor flip(F f);
> ```
>
> **Semantics**
>
> ``` c++
> assert(flip(f)(x, y, xs...) == f(y, x, xs...));
> ```

However, it would be nice to know what is the return type of the `operator()` of the `flip_adaptor`. In particular, I'm thinking about `auto` vs `decltype(auto)` for the return type. This is important, because it changes the expected behaviour of functions that return references, for example:

``` c++
std::string s = "foobar";
auto f = [&](auto const& ...) -> std::string& { return s; };
auto g = fit::flip(f);

std::string const& ref_to_s = g("don't", "care"); // is this holding a reference to `s`, or a (dangling) reference to a temporary created by `g`?
```

I understand this might present a challenge in terms of how to document this. If you want, you can take a look at how Hana specifies the behaviour of its functions using pseudo-code and lambdas, for example [here](http://boostorg.github.io/hana/group__group-functional.html#ga004f884cdbb85c2efe3383c1db450094).