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.
This is interesting because I thought left and right had to match, that is:
```rust
($($x:expr),+) => { $($x),+ };
// Valid   ^             ^
($($x:expr),+) => { $($x)-+ };
// Invalid ^             ^
// because left and right don't match
```