In fact, your issue (#3089) recalled me a kind of awkwardness of “union” operator and led to create this topic 
Well, as you said, the infobox is too small for clear explanation, so now I also think it’s better to improve the wiki. (But I’m too lazy to make visual example of these…)
Also, I welcome “difference” (or “subtract”) operator! To be honest, in almost all interactive use-case, I chose only “append lists” other than pairwise operators because they are too complicated.
However, I think these are essential for “text manipulation language.” And I guess difference operator may be useful when to split selections manually.