Documenting fetchTree is an exercise in frustration because of the sheer amount of stringly typed everything in it. I do not know which fields exist without reading the entirety of libfetchers. However, we can write something slightly perfunctory but at least perhaps somewhat helpful documentation-wise. Fixes: https://git.lix.systems/lix-project/lix/issues/609 Change-Id: I991391b53fcd69172dbc7efb9d384e62bc847b91