Remove the 'new in the next release' notes: everything is in 3.1.0.
Document pixel masks: painting and erasing, the mask shortcuts, and the COCO and PNG mask formats.
Document the new shortcuts (open folder, export in the last format, copy/paste, moving with the arrow keys, category number keys, show selected) and remove the notes for features released in 3.0.0.
Update the user manual to the current version.