bpo-25735: math.factorial doc should mention integer return type (GH-6420)

(cherry picked from commit 4612671df2742eade8ecf8003a6ce1247973c135)

Co-authored-by: Akshay Sharma <akshay.sharma09695@gmail.com>
diff --git a/Doc/library/math.rst b/Doc/library/math.rst
index b06b054..fd96d7a 100644
--- a/Doc/library/math.rst
+++ b/Doc/library/math.rst
@@ -50,7 +50,7 @@
 
 .. function:: factorial(x)
 
-   Return *x* factorial.  Raises :exc:`ValueError` if *x* is not integral or
+   Return *x* factorial as an integer.  Raises :exc:`ValueError` if *x* is not integral or
    is negative.