> I think it would be good if these two uses of "agglomerate" were more
> distinguished: feature agglomeration doesn't require agglomerative clustering,

Indeed, but in our experience agglomerative clustering is an excellent
clustering method candidate for feature agglomeration, because it is a
clustering method that is computationally efficient for a large number of
clusters. This is why we never implemented the generic object, that
doesn't work well in our experience.

> and is better placed under a dimensionality reduction discussion than under
> Hierarchical Clustering, IMO.

Indeed. Actually, I had added the main documentation entry in the
preprocessing documentation, along with PCA. The entry in the clustering
documentation is meant to serve only as a pointer. I tried to improve
things a bit in #445eee2 .