Difference: update documentation to reflect that selected variables must be manually redefined after reopening workflow file

[wvdvegte]

The Difference widget does not remember the variables selected for differentiation. Since this hasn't been resolved over three years since the last time it was reported (#243 in 2022), I assume this bug is too hard or too time-consuming to fix. How about including a warning in its documentation advising the user to include the variables to be selected at loading the workflow in the widget's description below the widget on the canvas?

[janezd]

The problem is not the complexity of the fix. This add-on doesn't have a dedicated maintainer. I made a round of fixes and updates in summer 2022, and nobody has really touched it since then.

Forgetting the selection seems to be my fault. I'll take a look as soon as I can, hopefully this week.

[wvdvegte]

Would it perhaps be an idea to include the functionality of Difference in Moving Transform, as a "subtraction" option?