Improve variadic templates section

* Use clearer identifier names.
* 'Currying' example is actually [partial application](http://en.wikipedia.org/wiki/Partial_application) (Note `std.functional.curry` was renamed `std.functional.partial`; see also https://github.com/D-Programming-Language/phobos/pull/2467).
* Simplify `partial` example.
* Minor fixes; move some text.
Please don't capitalize "Tuple" as it's too easily confused with std.typecons.Tuple