add use_scalevar_uncertainties to loading custom comvat docs

Created by: RoyStegeman

Different from what we originally said, there are different ways of combining custom covmats and covmats that are generated from pointprescirption instructions in the runcards and it's not either one or the other. The implementation of scalevariations is in general a bit messy so we may want to improve that, but it's not a priority right now so hopefully we can fix that at some point :crossed_fingers:.

Having said that - this bit of the docs was still outdated.