[SYCL][Doc] Deprecate reduction extension (#4941)

As discussed in #4831, we should officially deprecate the original reduction extension in favor of the equivalent SYCL 2020 reduction functionality.  This PR only makes the changes to the documentation -- adding deprecation notices to the implementation will be done in a separate PR.

Author: Pennycook

sycl/doc/extensions/Reduction/Reduction.md

@@ -1,6 +1,6 @@
 # SYCL(TM) Proposals: Reductions for ND-Range Parallelism
 
-**IMPORTANT**: This specification is a draft.
+**IMPORTANT**: The functionality introduced by this extension is deprecated in favor of the standard reduction functionality outlined in [Section 4.9.2 "Reduction variables"](https://www.khronos.org/registry/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:reduction) of the SYCL 2020 Specification, Revision 3.
 
 **NOTE**: Khronos(R) is a registered trademark and SYCL(TM) is a trademark of the Khronos Group, Inc.
 
@@ -17,6 +17,9 @@ It should also be noted that reductions are not limited to scalar values: the be
 # `reduction` Objects
 
 ```c++
+namespace sycl {
+namespace ext {
+namespace oneapi {
 template <class T, class BinaryOperation>
 unspecified reduction(accessor<T>& var, BinaryOperation combiner);
 
@@ -34,12 +37,18 @@ unspecified reduction(span<T, Extent> var, BinaryOperation combiner);
 
 template <class T, class Extent, class BinaryOperation>
 unspecified reduction(span<T, Extent> var, const T& identity, BinaryOperation combiner);
+}
+}
+}
 ```
 
 The exact behavior of a reduction is specific to an implementation; the only interface exposed to the user is the set of functions above, which construct an unspecified `reduction` object encapsulating the reduction variable, an optional operator identity and the reduction operator.  For user-defined binary operations, an implementation should issue a compile-time warning if an identity is not specified and this is known to negatively impact performance (e.g. as a result of the implementation choosing a different reduction algorithm).  For standard binary operations (e.g. `std::plus`) on arithmetic types, the implementation must determine the correct identity automatically in order to avoid performance penalties.
 
 If an implementation can identify the identity value for a given combination of accumulator type `AccumulatorT` and function object type `BinaryOperation`, the value is defined as a member of the `known_identity` trait class:
 ```c++
+namespace sycl {
+namespace ext {
+namespace oneapi {
 template <typename BinaryOperation, typename AccumulatorT>
 struct known_identity {
   static constexpr AccumulatorT value;
@@ -48,11 +57,17 @@ struct known_identity {
 // Available if C++17
 template <typename BinaryOperation, typename AccumulatorT>
 inline constexpr AccumulatorT known_identity_v = known_identity<BinaryOperation, AccumulatorT>::value;
+}
+}
+}
 ```
 
 Whether `known_identity<BinaryOperation, AccumulatorT>::value` exists can be tested using the `has_known_identity` trait class:
 
 ```c++
+namespace sycl {
+namespace ext {
+namespace oneapi {
 template <typename BinaryOperation, typename AccumulatorT>
 struct has_known_identity {
   static constexpr bool value;
@@ -61,6 +76,9 @@ struct has_known_identity {
 // Available if C++17
 template <typename BinaryOperation, typename AccumulatorT>
 inline constexpr bool has_known_identity_v = has_known_identity<BinaryOperation, AccumulatorT>::value;
+}
+}
+}
 ```
 
 The dimensionality of the `accessor` passed to the `reduction` function specifies the dimensionality of the reduction variable: a 0-dimensional `accessor` represents a scalar reduction, and any other dimensionality represents an array reduction.  Specifying an array reduction of size N is functionally equivalent to specifying N independent scalar reductions.  The access mode of the accessor determines whether the reduction variable's original value is included in the reduction (i.e. for `access::mode::read_write` it is included, and for `access::mode::discard_write` it is not).  Multiple reductions aliasing the same output results in undefined behavior.
@@ -70,6 +88,9 @@ The dimensionality of the `accessor` passed to the `reduction` function specifie
 # `reducer` Objects
 
 ```c++
+namespace sycl {
+namespace ext {
+namespace oneapi {
 // Exposition only
 template <class T, class BinaryOperation, int Dimensions, /* implementation-defined */>
 class reducer
@@ -91,6 +112,9 @@ class reducer
 
 // other operators should be made available for standard functors
 template <typename T> auto& operator+=(reducer<T,std::plus<T>,0>&, const T&);
+}
+}
+}
 ```
 
 The `reducer` class is not user-constructible, and can only be constructed by an implementation given a `reduction` object.  The `combine` function uses the specified `BinaryOperation` to combine the `partial` result with the value held (or referenced) by an instance of `reducer`, and is the only way to update the reducer value for user-supplied combination functions.  Other convenience operators should be defined for standard combination functions (e.g. `+=` for `std::plus`).