Add registry documentation to Source spec

**Describe the bug**
The source spec was merged in https://github.com/knative/eventing/pull/1856 without registry information. We should add that.

 **Expected behavior**
The source spec contains registry-related information

 **To Reproduce**
Steps to reproduce the behavior.

 **Knative release version**
Hopefully 0.10, otherwise 0.11


/assign