bpo-26701: Add documentation for __trunc__ (GH-6022)

`int` fails back to `__trunc__` is `__int__` isn't defined, so cover
that in the docs.
diff --git a/Doc/library/math.rst b/Doc/library/math.rst
index 55eb41b..33aec57 100644
--- a/Doc/library/math.rst
+++ b/Doc/library/math.rst
@@ -203,7 +203,7 @@
 
    Return the :class:`~numbers.Real` value *x* truncated to an
    :class:`~numbers.Integral` (usually an integer). Delegates to
-   ``x.__trunc__()``.
+   :meth:`x.__trunc__() <object.__trunc__>`.
 
 
 Note that :func:`frexp` and :func:`modf` have a different call/return pattern