[SPIR-V][DOC] Update SPV_INTEL_function_pointers

[MrSidims]

OpFunctionPointerINTEL was renamed to OpConstantFunctionPointerINTEL

SPIR-V to LLVM IR Translator update: https://github.com/KhronosGroup/SPIRV-LLVM-Translator/pull/1265 SPIR-V Headers update: https://github.com/KhronosGroup/SPIRV-Headers/pull/251 SPIR-V Tools update: https://github.com/KhronosGroup/SPIRV-Tools/pull/4613

Signed-off-by: Dmitry Sidorov [email protected]

[AlexeySachkov]

@MrSidims, @AlexeySotkin, we need to incorporate #2254 into this PR: we need to list Alex Bezzubikov as a contributor and copy issues raised in that PR into the spec. The rest should come from this PR, i.e. ultimately we should have OpConstant* spelling for new instructions, but we shouldn't forget about contributors and discussions which happened during an intermediate step (OpConst* spelling)