fixup: include link to Buffer.byteLength()

doc/api/http.md

@@ -1713,9 +1713,10 @@ const server = http.createServer((req, res) => {
 });
 ```
 
-`Content-Length` is given in bytes, not characters. Node.js does not check
-whether `Content-Length` and the length of the body which has been transmitted
-are equal or not.
+`Content-Length` is given in bytes, not characters. Use
+[`Buffer.byteLength()`][] to determine the length of the body in bytes. Node.js
+does not check whether `Content-Length` and the length of the body which has
+been transmitted are equal or not.
 
 Attempting to set a header field name or value that contains invalid characters
 will result in a [`TypeError`][] being thrown.
@@ -2385,6 +2386,7 @@ not abort the request or do anything besides add a `'timeout'` event.
 [`'response'`]: #http_event_response
 [`'upgrade'`]: #http_event_upgrade
 [`Agent`]: #http_class_http_agent
+[`Buffer.byteLength()`]: buffer.html#buffer_class_method_buffer_bytelength_string_encoding
 [`Duplex`]: stream.html#stream_class_stream_duplex
 [`TypeError`]: errors.html#errors_class_typeerror
 [`URL`]: url.html#url_the_whatwg_url_api