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.
You can use other separators (as long as they are not ambiguous with operators like `+`, `*`, `-`, etc)

For example: `($($x:expr) and +)` matches `(1 and 2 and 3)`