move cluster size restrictions for cl_khr_subgroup_clustered_reduce

[bashbaug]

fixes #1534

Instead of documenting cluster size restrictions in the section header, describe the restrictions in the description for each of the functions. This aligns with the way cluster size restrictions are documented for cl_khr_subgroup_rotate and makes the restriction easier to spot.

[karolherbst]

Mhh looks good. I just wonder if we want to converge the wording, because for rotate it says "clustersize must be an integer constant expression and a power of two, smaller than or equal to the maximum sub-group size, otherwise the behavior is undefined."

Picking either wording, or leaving it as is is fine with me.

[bashbaug]

I just wonder if we want to converge the wording

Sure, that's easy enough to do.

I've gone with the clustered reduce wording for now instead of the rotate wording because we usually use "must" to describe conditions that are statically checked (or at least, checkable), and that isn't the case for any of these conditions.

CC @kpet for awareness.