gh-84095: Fill documentation gap regarding user-defined objects. (GH-96574)

Author: vsajip

Doc/library/logging.config.rst

@@ -534,6 +534,25 @@ mnemonic that the corresponding value is a callable.
        The ``filters`` member of ``handlers`` and ``loggers`` can take
        filter instances in addition to ids.
 
+You can also specify a special key ``'.'`` whose value is a dictionary is a
+mapping of attribute names to values. If found, the specified attributes will
+be set on the user-defined object before it is returned. Thus, with the
+following configuration::
+
+    {
+      '()' : 'my.package.customFormatterFactory',
+      'bar' : 'baz',
+      'spam' : 99.9,
+      'answer' : 42,
+      '.' {
+        'foo': 'bar',
+        'baz': 'bozz'
+      }
+    }
+
+the returned formatter will have attribute ``foo`` set to ``'bar'`` and
+attribute ``baz`` set to ``'bozz'``.
+
 
 .. _logging-config-dict-externalobj: