tutorial: add note about copy being the default fallback when no refl…

…inks supported

per #579

static/docs/tutorial/define-ml-pipeline.md

@@ -91,9 +91,11 @@ copy) might take a few minutes.
 
 DVC was designed with large data files in mind. This means gigabytes or even
 hundreds of gigabytes in file size. Instead of copying files from cache to
-workspace, DVC can create reflinks or other file link types. (See
-[File link types](/docs/user-guide/large-dataset-optimization#file-link-types-for-the-dvc-cache)
-.)
+workspace, DVC can create reflinks or other file link types.
+
+> When reflinks are not supported, DVC defaults to copying files to avoid
+> problems with other file link types, but these can be enabled easily. See
+> [File link types](/docs/user-guide/large-dataset-optimization#file-link-types-for-the-dvc-cache
 
 Creating file links is a quick file system operation. So, with DVC you can
 easily checkout a few dozen files of any size. A file link prevents you from