Issue #2138: Add math.factorial().
diff --git a/Doc/library/math.rst b/Doc/library/math.rst
index b98f164..d6e0205 100644
--- a/Doc/library/math.rst
+++ b/Doc/library/math.rst
@@ -42,6 +42,10 @@
 
    Return the absolute value of *x*.
 
+.. function:: factorial(x)
+
+   Return *x* factorial.  Raises :exc:`ValueError` if *x* is not intergral or
+   is negative.
 
 .. function:: floor(x)