MapProperty Use mapping function with existing object

[johnnyelwailer]

Is your feature request related to a problem? Please describe.
According to the docs I can define user mapping function allowing me to use the existing object: https://mapperly.riok.app/docs/configuration/existing-target/

E.g. in my case I tried this:

private void MapDocuments(DocumentDto[] target, [MappingTarget] ICollection<Document> source)
{
    ...
}

Then the MapProperty attribute allows us to define a custom mapper function like:

[MapProperty(nameof(MyEntityDto.Documents), nameof(MyEntity.Documents), Use = nameof(MapDocuments))]
public partial void Map(MyDto dto, [MappingTarget] MyEntity entity);

However above use case fails with the error: "RMG061: The referenced mapping named MapDocuments was not found"

Describe the solution you'd like
MapProperty should allow us to use void methods that require the existing object.

Additonally:
In my specific case above I am using collection types. Initially I expected the MapDocuments method to be automaticly used as a UserMapping, without actually declaring MapProperty, but it never used my method, even when using the classic signature: ICollection<Document > Map(DocumentDto[] dtos) {...}

Additional context
Not sure if it's a feature or bug.

[latonz]

Thanks for bringing this up, this is not yet supported...