#16057: Clarify why the base method default is called in custom encoders.

Original patch by Kushal Das.
diff --git a/Doc/library/json.rst b/Doc/library/json.rst
index 7b69c24..58eee18 100644
--- a/Doc/library/json.rst
+++ b/Doc/library/json.rst
@@ -83,6 +83,7 @@
     ...     def default(self, obj):
     ...         if isinstance(obj, complex):
     ...             return [obj.real, obj.imag]
+    ...         # Let the base class default method raise the TypeError
     ...         return json.JSONEncoder.default(self, obj)
     ...
     >>> json.dumps(2 + 1j, cls=ComplexEncoder)
@@ -431,6 +432,7 @@
                 pass
             else:
                 return list(iterable)
+            # Let the base class default method raise the TypeError
             return json.JSONEncoder.default(self, o)