Add links from builtins module docs to built-in functions and constants docs
diff --git a/Doc/library/constants.rst b/Doc/library/constants.rst
index 80e792c..0db8337 100644
--- a/Doc/library/constants.rst
+++ b/Doc/library/constants.rst
@@ -1,3 +1,5 @@
+.. _built-in-consts:
+
 Built-in Constants
 ==================