I just watched https://developer.apple.com/videos/play/wwdc2021/10166/ to learn about docC, and it looks pretty much like what folks have been doing in other languages for a long time.

For example R developers usually document functions, data, etc using Roxygen in-line comments and they use vignettes for tutorials and the like. An additional advantage of R is that it checks that the documented function arguments line up with the code function arguments (at least by name -- it has no way to judge whether what the developer makes sense, of course).

I'm not saying that this is not useful, because I think it is, very, useful. It's just that this seem more like catching up than leading, at least in the broad strokes.