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 tried to generalize to `|`, `&`, and `;` but `|&` did bitmath and `;` possibly failed to parenthesis as before. I asked on IRC but couldn't get it working.