lots of warnings in documentation - unclear class reference

When I build the documentation, I get a lot of these errors:

```
~/icalendar/src/icalendar/cal/__init__.py:docstring of icalendar.cal.Component.related_to:1: WARNING: more than one target found for cross-reference 'vUri': icalendar.vUri, icalendar.prop.vUri [ref.python]
```

I would like to see none of them.