doc: add return type of readline.createInterface

PR-URL: #37600
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: RaisinTen

doc/api/readline.md

@@ -601,6 +601,7 @@ changes:
     **Default:** `500`.
   * `tabSize` {integer} The number of spaces a tab is equal to (minimum 1).
     **Default:** `8`.
+* Returns: {readline.Interface}
 
 The `readline.createInterface()` method creates a new `readline.Interface`
 instance.