Of the packages we’ve looked at, we consider Krita to have the best documentation available overall, particularly when considered as, specifically, an art package. The official website contains a full online user manual. The tutorials selection is also excellent. Searching around on YouTube shows that it’s well supported in the area of screencast tutorials made by the wider community.
The Inkscape website features a full online manual, a beginner’s guide (downloadable) and various illustrated tutorials. Searching around on YouTube revealed a healthy selection of tutorials covering most areas of the software. Due to its inherently technical nature, you’ll almost certainly need these resources.