Migration Guide: document the clustering improvement regarding write-through caches

Per https://developer.jboss.org/wiki/ClusteringChangesInWildFly8 and https://issues.jboss.org/browse/EAP7-329?focusedCommentId=13249669&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13249669, the new web session clustering implementation in EAP 7 works correctly with invalidation caches, unlike EAP 6. Write-through cache stores are typically used in concert with invalidation caches. This improvement should be documented in the Migration Guide.