[SYCL][NFC] Move SYCL_EXT_ONEAPI_USER_DEFINED_REDUCTIONS to experimental (#7850)

Author: dm-vodopyanov

sycl/doc/extensions/proposed/sycl_ext_oneapi_user_defined_reductions.asciidoc renamed to sycl/doc/extensions/experimental/sycl_ext_oneapi_user_defined_reductions.asciidoc

@@ -19,7 +19,7 @@
 == Notice
 
 [%hardbreaks]
-Copyright (C) 2022 Intel Corporation.  All rights reserved.
+Copyright (C) 2022-2023 Intel Corporation.  All rights reserved.
 
 Khronos(R) is a registered trademark and SYCL(TM) and SPIR(TM) are trademarks
 of The Khronos Group Inc.  OpenCL(TM) is a trademark of Apple Inc. used by
@@ -44,11 +44,12 @@ This extension also depends on the following other SYCL extensions:
 
 == Status
 
-This is a proposed extension specification, intended to gather community
-feedback.  Interfaces defined in this specification may not be implemented yet
-or may be in a preliminary state.  The specification itself may also change in
-incompatible ways before it is finalized.  *Shipping software products should
-not rely on APIs defined in this specification.*
+This is an experimental extension specification, intended to provide early
+access to features and gather community feedback.  Interfaces defined in this
+specification are implemented in {dpcpp}, but they are not finalized and may
+change incompatibly in future versions of {dpcpp} without prior notice.
+*Shipping software products should not rely on APIs defined in this
+specification.*
 
 == Overview