Add caveat about non-availability of sys.abiflags on Windows

Author: zklaus

peps/pep-0780.rst

@@ -199,7 +199,8 @@ For example, ``"32-bit" in sys.abi_features`` is much more expressive than the
 current standard test of comparing ``sys.maxsize`` with  ``2**32``, which can
 be found more than ten thousand times on GitHub.
 If one wants to determine whether the interpreter is a debug build, there is
-currently no standardized, cross platform way to do so.
+currently no standardized, cross platform way to do so, and ``sys.abiflags`` is
+not available on Windows.
 Hence, the `ABI Features`_ listed above are added to the Python standard
 library.