- add regression test for #4939
- add `[BindProperty]` doc comments
- add `<remarks>` to `BinderType` properties that recommend setting `BindingSource` in some cases
smaller issues:
- catch invalid `BinderType` values up front
- complete `BindingSource.ModelBinding` implementation: `IValueProvider` filtering was faulty
nits:
- accept VS suggestions e.g. remove unused variables
- "model binder" -> `<see cref="IModelBinder" /> implementation` in some doc comments
