`init="kmeans++"` is not documented as a valid option in KMeans

[aamijar]

https://github.com/rapidsai/cuml/blob/branch-25.12/python/cuml/cuml/cluster/kmeans.pyx#L55

I found that there is actually a kmeans++ initialization method in our KMeans implementation which works better than the other options for my use case, but it is not in the doc string.

I also believe this is not handled correctly in cuml.accel since scalable-k-means++ and kmeans++ are treated the same

https://github.com/rapidsai/cuml/blob/branch-25.12/python/cuml/cuml/cluster/kmeans.pyx#L418-L419