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.
Python's range in [library](https://docs.python.org/3/library/functions.html?highlight=range#func-range) and [tutorial](https://docs.python.org/3/tutorial/controlflow.html#the-range-function).