`kernel_handler` is underspecified

[4.9.5.3. Reading the value of a specialization constant from device code](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:spec-constants.device-code) provides a synopsis of `kernel_handler` class, but there are a few details which are missing, like:
- it it allowed for a user to manually construct such an object and pass it around?
- examples suggest that it is at least copy-constructible, but there is no explicit guarantee for that; can it be copy-assigned, for example?

Start with section 4.9.5.3, “Reading the value of a specialization constant from device code,” in the linked SYCL 2020 specification. Review the kernel_handler synopsis and the issue's questions about manual construction, copying, and assignment. Done means the specification explicitly resolves those object-semantics questions.

Written by the indexing model from the issue text.