The cl_intel_subgroups extension is no longer authored in plain text.

Please refer to the HTML extension specification instead,
which may be found here:

https://www.khronos.org/registry/OpenCL/extensions/intel/cl_intel_subgroups.html

The asciidoc source for this extension specification may
be found here:

https://github.com/KhronosGroup/OpenCL-Docs/blob/master/extensions/cl_intel_subgroups.asciidoc