I've found out how to customize this web service. It is defined in spring-wsInternalContext.xml.
The implementation class is com.jivesoftware.community.follow.rest.impl.FollowingServiceImpl that implements the interface com.jivesoftware.community.follow.rest.FollowingService.
This interface defines the path /follows that is necessary to call this service.
I'm still wondering why it isn't documented, but probably because it's an internal service.