Align descriptions of work group and subgroup scans and reductions

See related review comments:

* https://github.com/KhronosGroup/OpenCL-Docs/pull/560#pullrequestreview-589923619
* https://github.com/KhronosGroup/OpenCL-Docs/pull/560#issuecomment-785162394

>> I'm OK with this change, but it'd be nice to more closely align the descriptions of the work-group scans and reductions with the subgroup scans and reductions eventually.

> I would agree. In fact, generally I think it would be good to align the descriptions of workgroup and subgroup functions, and also things like half (cl_khr_fp16), float and double (cl_khr_fp64) functions. Of course, I prefer not to do all of this in one big PR as it would be difficult to review, would suffer from merge conflicts as other parts of the spec get changed over time, and wouldn't allow sharing of any of the workload.

Start with the linked review comments on pull request 560 and compare the descriptions of work-group and subgroup scans and reductions in the OpenCL documentation sources. Determine the specific wording that should be aligned, then confirm that the resulting descriptions are consistent without expanding the work to unrelated function families.

Written by the indexing model from the issue text.