doc: response.write ignores body in some cases

PR-URL: https://github.com/nodejs/node/pull/12314
Fixes: https://github.com/nodejs/node/issues/8057


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>