bpo-37315: Deprecate accepting floats in math.factorial(). (GH-14147)

diff --git a/Doc/library/math.rst b/Doc/library/math.rst
index ff937d2..bfce41a 100644
--- a/Doc/library/math.rst
+++ b/Doc/library/math.rst
@@ -71,6 +71,9 @@
    Return *x* factorial as an integer.  Raises :exc:`ValueError` if *x* is not integral or
    is negative.
 
+   .. deprecated:: 3.9
+      Accepting floats with integral values (like ``5.0``) is deprecated.
+
 
 .. function:: floor(x)