Document `stat` arg to `setHeaders` option

See https://github.com/expressjs/serve-static/issues/21
Would you be wiling to move it up to the https://github.com/expressjs/serve-static#setheaders section instead of just adding a dangling variable in the example (or, maybe make use of the `stat` variable somehow in the example)? It should at least be in the `setHeaders` doc section, though :)