To support the SYCL-Graph extension on an OpenCL backend, we currently
only require the presence of the `cl_khr_command_buffer` extension. This
PR introduces an extra requirement on the
[CL_COMMAND_BUFFER_SIMULTANEOUS_USE_KHR](https://registry.khronos.org/OpenCL/specs/3.0-unified/html/OpenCL_API.html#CL_COMMAND_BUFFER_SIMULTANEOUS_USE_KHR) capability being present.

This is based on the [graph execution
wording](https://github.com/intel/llvm/blob/sycl/sycl/doc/extensions/experimental/sycl_ext_oneapi_graph.asciidoc#765-new-handler-member-functions)
on the definition of `handler::ext_oneapi_graph()` that:

>  Only one instance of graph will execute at any time. If graph is submitted multiple times, dependencies are automatically added by the runtime to prevent concurrent executions of an identical graph.

Such usage results in multiple calls by the SYCL runtime to
`urEnqueueCommandBufferExp` with the same UR command-buffer and event
dependencies to prevent concurrent execution. Without support for
simultaneous-use the OpenCL adapter code cannot guarantee that the
first command-buffer submission has finished execution before it
makes following `clEnqueueCommandBufferKHR` calls with the `cl_event`
decencies. If the first submission is still executing, then an error
will be reported.

Workarounds like adding blocking host waits to the OpenCL UR adapter
are possible, but requiring simultaneous use reflects the vendor
requirements as they are for the currently implementation.

I've tried to document this all in the UR spec and SYCL-Graph design
docs, which also includes a couple of cleanups I found along the way.