doc: response.write ignores body in some cases

doc/api/http.md

@@ -1161,6 +1161,10 @@ it will switch to implicit header mode and flush the implicit headers.
 This sends a chunk of the response body. This method may
 be called multiple times to provide successive parts of the body.
 
+Note that in the `http` module, the response body is omitted when the
+request is a HEAD request. Similarly, the `204` and `304` responses
+_must not_ include a message body.
+
 `chunk` can be a string or a buffer. If `chunk` is a string,
 the second parameter specifies how to encode it into a byte stream.
 By default the `encoding` is `'utf8'`. `callback` will be called when this chunk