Razor Pages ConfigureFilter does not work on Areas

### Is your feature request related to a problem? Please describe.
Configuring Filters for Razor Pages does not work on areas.
i.e. the following will not work for areas:
`options.Conventions.ConfigureFilter(model => new AdminDomainFilterFactory(model));`

### Describe the solution you'd like
I think it should be added to the documentation.
that this is not possible.

Currently a workaround would be adding that:
```
public static IPageApplicationModelConvention ConfigureAreaFilter(
this PageConventionCollection conventions,
string areaName,
Func factory)
{
if (areaName == null)
throw new ArgumentNullException(nameof(areaName));
if (conventions == null)
throw new ArgumentNullException(nameof (conventions));
if (factory == null)
throw new ArgumentNullException(nameof (factory));
return conventions.AddAreaFolderApplicationModelConvention(areaName, "/", (Action) (model => model.Filters.Add(factory(model))));
}
```

a better solution of course would be if it would work on **all** areas

which is possible by implementing a `IPageApplicationModelConvention` that does not filter out the areaName.