The unit is the kilometer and is consistent with what is done in Lucene.

Glad to hear it. From what I see in org.hibernate.search.spatial.impl.Point.getDistanceTo(double, double), the unit used in Lucene is indeed the KM.
So we only have to add this information to the javadoc of org.hibernate.search.engine.ProjectionConstants.SPATIAL_DISTANCE and org.hibernate.search.elasticsearch.ElasticsearchProjectionConstants.SPATIAL_DISTANCE.