doc: note that zlib.flush acts after pending writes

Describe that `zlib.flush()` may wait for pending writes and
until output is being read from the stream.

Fixes: https://github.com/nodejs/node/issues/3782
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>