Add more explanations for all callback analysis quantities

[pnavaro]

The callback_analysis feature is very helpful for analyzing model precision, but we could not find explanations for some quantities such as ∫U for instance. These quantities could be clarified in the documentation of the equations.

JOSS review #9361

[JoshuaLampert]

I agree that this needs to be better documented. Should be resolved with #261. Thanks!