State that node class has common reference semantics

SYCL [common reference
semantics](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:reference-semantics)
define copy, move, destruction, and equality requirements for runtime
classes.

Issue #22

sycl/doc/extensions/proposed/sycl_ext_oneapi_graph.asciidoc

@@ -312,11 +312,15 @@ public:
 
 === Node
 
+:crs: https://www.khronos.org/registry/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:reference-semantics
+
 Node is a class that encapsulates tasks like SYCL kernel functions, device
 memory allocations/frees, or host tasks for deferred execution. A graph has to
 be created first, the structure of a graph is defined second by adding nodes and
 edges.
 
+The `node` class provides the {crs}[common reference semantics].
+
 [source,c++]
 ----
 namespace sycl::ext::oneapi::experimental {
@@ -326,8 +330,6 @@ namespace sycl::ext::oneapi::experimental {
 
 === Graph
 
-:crs: https://www.khronos.org/registry/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:reference-semantics
-
 This extension adds a new `command_graph` object which follows the
 {crs}[common reference semantics] of other SYCL runtime objects.