Improve documentation for auto add replicas and trigger documentation

After SOLR-12715, the documentation (and user story) around the auto add replicas feature is inconsistent. This is because even though we can now add new replicas automatically on new nodes, the autoAddReplicas=true parameter on the create collection API only moves replicas around to replace the lost replicas. We should try to fix our documentation first to describe these two features in a coherent way and open another issue, if needed, to change the behavior of autoAddReplicas parameter in Solr 8.0.