doc: specify default encoding in writable.write

Refs: #33715

PR-URL: #33765
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/stream.md

@@ -576,7 +576,7 @@ 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. **Default:** `'utf8'`
 * `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