Explain square unity bonus ingame

I was quite surprised to learn that the decorative looking squares provide a unity bonus. This adjacency bonus is seemingly nowhere explained or hinted at ingame. As per the title, explain this ingame, e.g. in the square object/tech description or some settlement screen tooltip.

It *kinda* is, but I only realised that the "decoration bonus" was a Unity bonus only after building one. So I do agree it should be more clear.

I was far too many hours into playing before I stumbled on this bonus.
It for sure needs to be more clear and upfront early in the game.
Shame we don't have a real place to read about the game, wiki is useless and months out of date.