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.
Found it [here](http://doc.rust-lang.org/rust.html#macro-by-example)

"Valid designators are item, block, stmt, pat, expr, ty (type), ident, path, matchers (lhs of the => in macro rules), tt (rhs of the => in macro rules)."

I don't think we should describe all of them, just a few, and maybe mention all the designators.

I have only used a few of them: `ty`, `expr` and `ident` are the most common