doc: found better use-case for using `rstream.unshift`

In the example before `unshift()` does the same as `push()` we are anyway
having managing the first chunk. But by using the `unshift()` on the source
we have can reread our chunk (now as body) on the next `readable` event.
Source: https://groups.google.com/forum/?fromgroups=#!topic/nodejs/hTA91YugVAk