rgb is an old expression from C2 that was deprecated years ago, so you'll most likely never encounter it unless importing an old project. One of the reasons it was deprecated is it is inconsistent with the new expressions, and removing it was meant to avoid this kind of confusion, but it still comes up with old projects.
I think the manual entry is clear if you note that rgb is not in the list as it's deprecated. The four existing expressions are consistent: rgbEx and rgba work 0-100, and rgbEx255 and rgba255 work 0-255. In other words if it says 255 then it's 0-255, otherwise it's 0-100. But the old rgb expression breaks that rule: it uses 0-255. But it's hidden in Construct 3.