bpo-34512: Document platform-specific strftime() behavior for non-ASCII format strings (GH-8948)

(cherry picked from commit 1cffd0ee)
Co-authored-by: Alexey Izbyshev <izbyshev@ispras.ru>

Doc/library/datetime.rst

@@ -1611,6 +1611,12 @@ calls the platform C library's :func:`strftime` function, and platform
 variations are common.  To see the full set of format codes supported on your
 platform, consult the :manpage:`strftime(3)` documentation.
 
+For the same reason, handling of format strings containing Unicode code points
+that can't be represented in the charset of the current locale is also
+platform-dependent. On some platforms such code points are preserved intact in
+the output, while on others ``strftime`` may raise :exc:`UnicodeError` or return
+an empty string instead.
+
 The following is a list of all the format codes that the C standard (1989
 version) requires, and these work on all platforms with a standard C
 implementation.  Note that the 1999 version of the C standard added additional