doc: specify default encoding in writable.write

Refs: nodejs#33715

doc/api/stream.md

@@ -589,7 +589,8 @@ changes:
  not operating in object mode, `chunk` must be a string, `Buffer` or
  `Uint8Array`. For object mode streams, `chunk` may be any JavaScript value
  other than `null`.
-* `encoding` {string} The encoding, if `chunk` is a string
+* `encoding` {string} The encoding, if `chunk` is a string. Defaults to `utf8`
+ if a falsey value is passed.
 * `callback` {Function} Callback for when this chunk of data is flushed
 * Returns: {boolean} `false` if the stream wishes for the calling code to
  wait for the `'drain'` event to be emitted before continuing to write