Clarify in models README's that the IDs in the "model's outputs" are not the real IDs

[reyammer]

In the models' README, we have a table with the list of possible outputs and IDs. These IDs are NOT the IDs / indexes of the model's softmax layer (undefined is not a real model output, but it is a possible value for dl.output). We should probably clarify this nit, as it may lead to false assumptions. The real source of truth for ID <=> content type is the model's config target labels.