Selection combining operator "u: union" is little bit misleading

[Delapouite]

@murashitas

In fact, your issue (#3089) recalled me a kind of awkwardness of “union” operator and led to create this topic

This commit resolves the ambiguity: Add unambiguous titles for infobox "enter combining operator" · mawww/kakoune@2148b84 · GitHub

The next step that would really help about these combinators is my proposal for the mark highlighter: Add mark highlighter by Delapouite · Pull Request #2461 · mawww/kakoune · GitHub