Fix documentation of stat parameter

[fkohrt]

stat accepts e.g. ggplot2::StatBin (class Stat), but not ggplot2::GeomBar (class Geom).

[teunbrand]

Thanks for preparing a PR for this. Unfortunately, this particular point is also addressed in #5127, so these PRs are conflicting but both solve the issue.

[teunbrand]

I do appreciate the effort going into this PR and want to thank you for that, but I think I'll close this one in favour #5127.