Update API v4 for opinion/cluster

[flooie]

The CourtListener API documentation for the Opinion endpoint incorrectly suggests that the citations field is available in the Opinion API response. However, this field is on the Cluster API, causing confusion for users trying to retrieve citation data for opinions.

We need to update it so users find citations on the cluster object.

See: https://www.courtlistener.com/help/api/rest/case-law/#opinion-endpoint