gh-103878: Document that the cancelled file dialog value is only falsy (GH-152429)

Its exact type (empty string, tuple, list or None) varies between
platforms and Tk versions, so advise testing the result for truth.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka

Doc/library/dialog.rst

@@ -159,7 +159,10 @@ listed below:
 The below functions when called create a modal, native look-and-feel dialog,
 wait for the user's selection, and return it.
 The exact return value depends on the function (see below); when the dialog is
-cancelled it is an empty string, an empty tuple, an empty list or ``None``.
+cancelled it is an empty string, an empty tuple or ``None``.
+The precise type of this empty value may vary between platforms and Tk
+versions, so test the result for truth rather than comparing it with a
+specific value.
 
 .. function:: askopenfile(mode="r", **options)
               askopenfiles(mode="r", **options)
@@ -168,7 +171,7 @@ cancelled it is an empty string, an empty tuple, an empty list or ``None``.
    :func:`askopenfile` returns the opened file object, or ``None`` if the
    dialog is cancelled.
    :func:`askopenfiles` returns a list of the opened file objects, or an empty
-   list if cancelled.
+   tuple if cancelled.
    The files are opened in mode *mode* (read-only ``'r'`` by default).
 
 .. function:: asksaveasfile(mode="w", **options)