bpo-36019: Use pythontest.net instead of example.com in network tests (GH-11941)

diff --git a/Doc/library/test.rst b/Doc/library/test.rst
index b5057c0..054521d 100644
--- a/Doc/library/test.rst
+++ b/Doc/library/test.rst
@@ -356,6 +356,10 @@
 
    Check for presence of docstrings.
 
+.. data:: TEST_HTTP_URL
+
+   Define the URL of a dedicated HTTP server for the network tests.
+
 
 
 The :mod:`test.support` module defines the following functions: