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 not broken for the reason you state "A list is never re-expanded". It's just that the expansion is not valid.

This `echo_broken!(1u, 2u, 3u)` expands to `1, 2, 3` which is not a valid expansion, because is not an expression.

But if you add parentheses to your macro definition, it'll work:

`($($x:expr),+) => { ($($x),+) };`

Now `echo_broken!(1, 2, 3)` expands to `(1, 2, 3)`, which is a valid expression (is a tuple).

I can't think of an useful macro, using this non-recursive form of expansion. You could do something like this:

`($($x:expr),+) => { $($x)-+ };`

Now `echo_broken!(3, 2, 1)` expands to `3 - 2 - 1`