[clang] [clang][SYCL] Diagnose variadic sycl_external function (PR #216393)

[Tom Honermann via cfe-commits]

https://github.com/tahonermann requested changes to this pull request.

Thanks, @schittir, this looks good. Just some minor requests for documentation markup corrections following the recent documentation migration to markdown and some comment updates.

https://github.com/llvm/llvm-project/pull/216393