doc: add full example for zlib.flush()

Add a full example using `zlib.flush()` for the common use
case of writing partial compressed HTTP output to the client.

PR-URL: https://github.com/nodejs/node/pull/6172


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Robert Jefe Lindstädt <robert.lindstaedt@gmail.com>