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.
~~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](http://rustbyexample.com/staging/macros.html) is more like a variadic function (it can have any arity, well, except 0-arity in this particular case).