gh-52008: Document that ctypes.CDLL can only observe errno from its own C runtime (GH-159239)

Author: ZeroIntensity

Doc/library/ctypes.rst

@@ -1579,7 +1579,10 @@ way is to instantiate :py:class:`CDLL` or one of its subclasses:
    :mod:`!ctypes` maintains a thread-local copy of the system's :data:`errno`
    variable; if you call foreign functions created with ``use_errno=True`` then the
    :data:`errno` value before the function call is swapped with the ctypes private
-   copy, the same happens immediately after the function call.
+   copy, the same happens immediately after the function call. This mechanism only
+   works for libraries that use the same C runtime library as the interpreter itself.
+   A library linked against a different C runtime (for example, a different CRT on
+   Windows) has its own :data:`errno`, which :mod:`!ctypes` cannot observe.
 
    The function :func:`ctypes.get_errno` returns the value of the ctypes private
    copy, and the function :func:`ctypes.set_errno` changes the ctypes private copy