#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 648ba3a..a3c6bb0 100644
--- a/Doc/library/json.rst
+++ b/Doc/library/json.rst
@@ -84,6 +84,7 @@
     ...     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)
     ...
     >>> dumps(2 + 1j, cls=ComplexEncoder)
@@ -452,6 +453,7 @@
                 pass
             else:
                 return list(iterable)
+            # Let the base class default method raise the TypeError
             return JSONEncoder.default(self, o)