Leftover tasks for module-level statics

_(extracted from https://github.com/HaxeFoundation/haxe/pull/8460)_

- [x] ~discuss metadata/extern for KModuleStatics class~ (moved to #9494)
- [x] adapt other targets to "flatten" module-statics classes
- [x] better error messages for module-level `public`/`static` keywords
- [ ] document resolution order (with and without import)
- [x] check/adjust dox generation
- dox should display modules to support this https://github.com/HaxeFoundation/dox/issues/268
- [ ] update the Haxe manual
- [ ] decide on naming for `KModuleStatics` in macro API