Issue #9542: Create PyUnicode_FSDecoder() function

It's a ParseTuple converter: decode bytes objects to unicode using
PyUnicode_DecodeFSDefaultAndSize(); str objects are output as-is.

 * Don't specify surrogateescape error handler in the comments nor the
   documentation, but PyUnicode_DecodeFSDefaultAndSize() and
   PyUnicode_EncodeFSDefault() because these functions use strict error handler
   for the mbcs encoding (on Windows).
 * Remove PyUnicode_FSConverter() comment in unicodeobject.c to avoid
   inconsistency with unicodeobject.h.
diff --git a/Misc/NEWS b/Misc/NEWS
index 28e0314..3388aec 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -12,6 +12,10 @@
 Core and Builtins
 -----------------
 
+- Issue #9542: Create PyUnicode_FSDecoder() function, a ParseTuple converter:
+  decode bytes objects to unicode using PyUnicode_DecodeFSDefaultAndSize();
+  str objects are output as-is.
+
 - Issue #9203: Computed gotos are now enabled by default on supported
   compilers (which are detected by the configure script).  They can still
   be disable selectively by specifying --without-computed-gotos.