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.
I thin the second sentence could become:

Here we'll implement a `range!` macro similar Python's range function, which is an overloaded function.

Probably a link to Python's range function will help.