[SYCL][Doc] Update to the test plan for free function kernels extension (#18235)

Updates based on feedback to the previous PR which has already been
merged with test plan for free function kernels extension.

Changes done:
- moved check related to aliases into unit-tests category.
- added two new checks into e2e category related to treating free
function kernel as device or host function.
- updated `Interaction with additional kernel properties` part.
- updated description for `get_kernel_id` what mean valid kernel
identifier in context of unit-testing.
- most of changes were related to refactoring text into consistent
column limit (which is 80) for markdown doc.


Previous PR: #17886
Docs:
https://github.com/intel/llvm/blob/sycl/sycl/doc/extensions/proposed/sycl_ext_oneapi_free_function_kernels.asciidoc

Author: dyniols