[SYCL][Graph] Add spec wording for graph-owned memory allocations

[Bensuo]

• Using sycl_ext_codeplay_async_memory_alloc extension (PR currently in review)
• Spec wording for graph support of the feature
• Usage guide guidance for library authors
• Usage guide examples of explicit and queue recording usage with and without mem pools

[Bensuo]

@gmlueck Not sure if you've done a detailed review of this already but it should be good to go now if you would like to take another look at it.

[gmlueck]

I will be OOO for the rest of the week, so I approved. I think you understand my concern in the remaining comments, and I trust that you will address them.

[Bensuo]

@intel/llvm-gatekeepers This should be good to merge now, thanks!