comment on use of data subset
@@ -350,7 +350,9 @@ Supervised UMAP on the Galaxy10SDSS dataset
The `Galaxy10SDSS dataset <https://astronn.readthedocs.io/en/latest/galaxy10sdss.html>`__
is a crowd sourced human labelled dataset of galaxy images,
which have been separated in to ten classes. Umap can
-learn an embedding that partially separates the data.
+learn an embedding that partially separates the data. To
+keep runtime small, UMAP is applied to a subset of the
+data.
.. code:: python3
