-# create_output_collections generates a new collection containing the
-# output of each successfully completed task, and returns the
-# portable_data_hash for the new collection.
-#
+# Create a collection by concatenating the output of all tasks (each
+# task's output is either a manifest fragment, a locator for a
+# manifest fragment stored in Keep, or nothing at all). Return the
+# portable_data_hash of the new collection.