Add impl-detail block for PyLong_FromLong docs

Author: skirpichev

Doc/c-api/long.rst

@@ -40,9 +40,11 @@ distinguished from a number.  Use :c:func:`PyErr_Occurred` to disambiguate.
 
    Return a new :c:type:`PyLongObject` object from *v*, or ``NULL`` on failure.
 
-   The current implementation keeps an array of integer objects for all integers
-   between ``-5`` and ``256``. When you create an int in that range you actually
-   just get back a reference to the existing object.
+   .. impl-detail::
+
+      The CPython keeps an array of integer objects for all integers
+      between ``-5`` and ``256``.  When you create an int in that range
+      you actually just get back a reference to the existing object.
 
 
 .. c:function:: PyObject* PyLong_FromUnsignedLong(unsigned long v)