gh-127805: Clarify Formatter initialization in logging.rst

Author: Uvi-12

Doc/library/logging.rst

@@ -562,7 +562,7 @@ 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 :class:`Formatter` for this handler to *fmt*. The *fmt* argument must be an initialized :class:`Formatter` instance.
 
 
    .. method:: Handler.addFilter(filter)