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.
I didn't ;-) I just moved a paragraph from below. Capitalized *Tuple* is already used several times above here - see: https://github.com/D-Programming-Language/dlang.org/pull/639/files#diff-1f5f5aad09e141eabfbf4ee7a349b424R717.