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.
Moving https://github.com/japaric/rust-by-example/pull/185#discussion_r15197023 so it's not hidden. Easier to remember.
> ~~implemented~~ -> emulated. They are still macros, they are not variadic functions.
~~variadic~~ -> This particular example range! resembles to function overloading, not to a variadic function.

> My min! macro is more like a variadic function (it can have any arity, well, except 0-arity in this particular case).