Document pipeline transformations applied by `UiLayer`

From the signature of its single public method: https://github.com/DiamondLightSource/httomo/blob/c7e399d09f56a238e4c731c5f66857047f7f4752/httomo/ui_layer.py#L50

it's clear enough that `UiLayer` is used to produce a `Pipeline`.

However, transforming pipeline data from some external format (eg, YAML) into a `Pipeline` is not all that it does, it makes some additional changes. These additional changes should be documented somewhere (possibly in the class docstring).

The only transformation it applies I think is taking side output references and inserting appropriate `OutputRef` objects into the parameter dicts of the relevant method wrappers.