Remove implementation details from PyLongExport docs

Author: skirpichev

peps/pep-0757.rst

@@ -133,10 +133,11 @@ Export API
 
       Read-only array of unsigned digits. Can be ``NULL``.
 
-   If :c:member:`digits` not ``NULL``, a private field of the
-   :c:struct:`PyLongExport` structure stores a strong reference to the Python
-   :class:`int` object to make sure that that structure remains valid until
-   :c:func:`PyLong_FreeExport()` is called.
+
+If :c:member:`PyLongExport.digits` not ``NULL``, a private field of the
+:c:struct:`PyLongExport` structure stores a strong reference to the Python
+:class:`int` object to make sure that that structure remains valid until
+:c:func:`PyLong_FreeExport()` is called.
 
 
 .. c:function:: int PyLong_Export(PyObject *obj, PyLongExport *export_long)