Clarify which half types and builtins need cl_khr_fp16 and which do not.

Current spec sections of interest

- Spec requires cl_khr_fp16 for functions taking or returning half types as per https://registry.khronos.org/OpenCL/specs/3.0-unified/html/OpenCL_C.html#vector-data-load-and-store-functions.
- Specification for halfn requires cl_khr_fp16 as per footnote https://registry.khronos.org/OpenCL/specs/3.0-unified/html/OpenCL_C.html#_footnotedef_8

Also refer to discussion on CTS PR https://github.com/KhronosGroup/OpenCL-CTS/pull/1974

Start by comparing the OpenCL C vector data load/store section with the halfn footnote, then review the discussion on OpenCL-CTS PR 1974. Document which half types and builtins require cl_khr_fp16 and which do not, with the specification wording made consistent.

Written by the indexing model from the issue text.