doc: clearify when does http module does chunked transfer encoding

Author: micromaomao

doc/api/http.md

@@ -883,13 +883,11 @@ added: v0.1.29
 * `callback` {Function}
 * Returns: {boolean}
 
-Sends a chunk of the body. By calling this method
-many times, a request body can be sent to a
-server. In that case, it is suggested to use the
-`['Transfer-Encoding', 'chunked']` header line when
-creating the request.
-
-Note that this module automatically encode data into chunked encoding.
+This method can be called multiple times. If no `Content-Length` is set, data
+will automatically be encoded in HTTP Chunked transfer encoding, so that
+server knows when the data ends. The `Transfer-Encoding: chunked` header will
+be added. Call `end()` when finish sending data so that node would send the
+trailer.
 
 The `encoding` argument is optional and only applies when `chunk` is a string.
 Defaults to `'utf8'`.