pythongh-127805: Clarify Formatter initialization in logging.rst. (pythonGH-127850)

Author: UV

Doc/library/logging.rst

@@ -562,7 +562,8 @@ subclasses. However, the :meth:`!__init__` method in subclasses needs to call
 
    .. method:: Handler.setFormatter(fmt)
 
-      Sets the :class:`Formatter` for this handler to *fmt*.
+      Sets the formatter for this handler to *fmt*.
+      The *fmt* argument must be a :class:`Formatter` instance or ``None``.
 
 
    .. method:: Handler.addFilter(filter)