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.
Could we change these suffixes from `i32` to `i`? The type is not important in this case, but I think that `10i` is easier to read than `10i32`. (Also, we don't need to suffix the second and third arguments)