explain how to create modal dialog (see #132)

nicegui/elements/dialog.py

@@ -11,7 +11,7 @@ class Dialog(Group):
     def __init__(self, *, value: bool = False):
         """Dialog
 
-        Creates a modal dialog.
+        Creates a dialog. By default it is non-modal. To make it modal, set `.props('persistent')` on the dialog object. 
 
         :param value: whether the dialog is already opened (default: `False`)
         """