add some explanation on form fields

[tharvik]

splitted from #850, @JulienVig proposed to

add some explanation for each field, maybe when hovering over an "i" or "?" icon as @tomasoignons had implemented I'm happy to help writing some descriptions

related improvements

we need to add more information for the model.json field, for example explain what is "TFJS", why we are using tensorflow.js, linking tfjs docs, where do download pre-trained models etc

I don't think anyone can pick a good value for "Maximum weights difference between each round"... Does it need a unit or naming how the difference is computed (e.g. "per Euclidean distance")?

What is the "Size of the noise added to the weights"?