[SYCL] Update wording around updating kernel/host code

- Make modifying kernel or host code in update undefined behaviour.

sycl/doc/extensions/proposed/sycl_ext_oneapi_graph.asciidoc

@@ -863,6 +863,8 @@ void
 update(const command_graph<graph_state::modifiable>& graph);
 ----
 
+:sycl-kernel-function: https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#sycl-kernel-function
+
 |Updates the executable graph node inputs & outputs from a topologically
 identical modifiable graph. A topologically identical graph is one with the
 same structure of nodes and edges, and the nodes added in the same order to
@@ -872,17 +874,16 @@ to update an executable graph, every node in the graph must be either a kernel
 command or a host-task.
 
 The only characteristics that can differ between two topologically identical
-graphs during an update are:
+graphs during an update are the arguments to kernel nodes. For example,
+the graph may capture different values for the USM pointers or accessors used
+in the graph. It is these kernels arguments in `graph` that constitute the
+inputs & outputs to update to.
 
-* The arguments to kernel nodes may be different between graphs. For example,
- the graph may capture different values for the USM pointers or accessors used
- in the graph. It is these kernels arguments in `graph` that constitute the
- inputs & outputs to update to.
+Differences in the following characteristics between two graphs during an
+update results in undefined behaviour:
 
-* The code that is executed in host-task nodes may be different between graphs,
- and the update will reflect the code from `graph` host-tasks nodes. However,
- any changes to executable code between kernel nodes are not counted as
- topologically identical and is an exception with error code `invalid`.
+* Modifying the native C++ callable of a host_task node.
+* Modifying the {sycl-kernel-function}[kernel function] of a kernel node.
 
 The effects of the update will be visible on the next submission of the
 executable graph without the need for additional user synchronization.