add JSDoc annotations to geoExtent

[k-yle]

I excluded this from #11389, because there's no elegant way to do it[^1]. geoExtent is used everywhere, so it would be very helpful to get proper auto-complete. The trade-off is a couple of ugly lines of JSDoc comments in the class constructor...

[^1]: besides from converting it to an ES6 class, but that would require a minor change to 52 files, which we don't want to do