Improve Documentation of kernel name system (generator / provider)

[danhoeflinger]

Improve both the user facing documentation of kernel names, and also internal documentation of our kernel name system.

Our public documentation needs to better describe that custom names must be unique, and/or that they follow the same rules as sycl kernel names.

It needs to be more clear in the code when to use __kernel_name_generator, __kernel_name_provider, and the what requirements are when using each. We could consider improving the names of these structures, as well as adding comments, and perhaps an RFC documenting the whole system for reference.