sync: Refer to go123 who now provides xsync.WorkGroup

https://godoc.org/lab.nexedi.com/kirr/go123/xsync#WorkGroup
go123@515a6d14

golang/_sync.pxd

 # cython: language_level=2
-# Copyright (C) 2019  Nexedi SA and Contributors.
+# Copyright (C) 2019-2020  Nexedi SA and Contributors.
 #                          Kirill Smelkov <kirr@nexedi.com>
 #
 # This program is free software: you can Use, Study, Modify and Redistribute
@@ -26,7 +26,9 @@
 
 See also https://golang.org/pkg/sync for Go sync package documentation.
 
-(*) not provided in Go version.
+(*) not provided in Go standard library, but package
+    https://godoc.org/lab.nexedi.com/kirr/go123/xsync
+    provides corresponding Go equivalents.
 """
 
 from golang cimport error, refptr

golang/sync.h

 #ifndef _NXD_LIBGOLANG_SYNC_H
 #define	_NXD_LIBGOLANG_SYNC_H
 
-// Copyright (C) 2018-2019  Nexedi SA and Contributors.
+// Copyright (C) 2018-2020  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -36,7 +36,9 @@
 //
 //  - `_makesema` and `_sema*` provide semaphore functionality(*).
 //
-// (*) not provided in Go version.
+// (*) not provided in Go standard library, but package
+//     https://godoc.org/lab.nexedi.com/kirr/go123/xsync
+//     provides corresponding Go equivalents.
 
 #include <golang/libgolang.h>
 #include <golang/context.h>

golang/sync.py

 # -*- coding: utf-8 -*-
-# Copyright (C) 2019  Nexedi SA and Contributors.
+# Copyright (C) 2019-2020  Nexedi SA and Contributors.
 #                          Kirill Smelkov <kirr@nexedi.com>
 #
 # This program is free software: you can Use, Study, Modify and Redistribute
@@ -26,7 +26,9 @@
 
 See also https://golang.org/pkg/sync for Go sync package documentation.
 
-(*) not provided in Go version.
+(*) not provided in Go standard library, but package
+    https://godoc.org/lab.nexedi.com/kirr/go123/xsync
+    provides corresponding Go equivalents.
 """
 
 from __future__ import print_function, absolute_import