Thread-safety

I saw there was a recent comment on this, so I think the WG would like to push this forward. Summarizing my thoughts ... I think we need a much larger change if we really want to clarify the thread safety guarantees of SYCL. I think John and I agreed in the comments above that we need to go through the APIs one-by-one and decide exactly what thread safety guarantees we expect.

If we are looking to merge this PR quickly as a step in the right direction, I'd be happy with this PR if we delete the non-normative note. I think the note (as written now) makes the wording more confusing, not less.

_Originally posted by @gmlueck in https://github.com/KhronosGroup/SYCL-Docs/issues/831#issuecomment-3246820598_

Start by reading the discussion referenced from issue 831 and the comments summarized here. The proposed work is to review SYCL APIs one by one and clarify their thread-safety guarantees, while the immediate alternative is to remove a non-normative note. Done would require an agreed scope and specification changes, but no files or tests are identified.

Written by the indexing model from the issue text.