Expand macro section on repetition operators and syntax.

This is to give more details on how macro repetition works and to address https://github.com/japaric/rust-by-example/issues/110#issuecomment-48104754.
Scratch that, this particular section is not about variadic macros, the section about `($x,)+` is more like variadic macros. And on a second thought, I think we should avoid the "variadic" word until we get real [variadic generics](https://github.com/rust-lang/rust/issues/10124) (if we ever get them).