doc: add return type of readline.createInterface

PR-URL: #37600
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Benjamin Gruenbaum <[email protected]>
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: James M Snell <[email protected]>

Author: RaisinTen

Diff for: 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.