Documentation on the relevance of some diagnostic differences

Some variables are not necessarily directly comparable, such as radar reflectivity which is on a log scale. So adding an issue to decide whether we need ways to choose which variables to exclude from this, or at least have documentation which captures this.

No files, tests, or entry points are named. Start by locating the diagnostic comparison documentation and the handling of variables such as log-scaled radar reflectivity. Done means documenting the relevance of these differences and clarifying whether users need a way to exclude selected variables.

Written by the indexing model from the issue text.