StackLayer, extend doc

Author: albertz

returnn/tf/layers/basic.py

@@ -5038,6 +5038,9 @@ def get_out_data_from_opts(cls, axis, enforce_batch_dim_axis=None, allow_no_op=F
 class StackLayer(LayerBase):
   """
   Stacks multiple inputs together using :func:`tf.stack`.
+  This creates a new dimension for the stack.
+
+  For concatenation (in feature dimension), see :class:`CopyLayer`.
   """
   layer_class = "stack"