[DOCS] Improve documentation on CCR alias replication

https://github.com/elastic/elasticsearch/pull/41815 introduced replication for aliases in the CCR context. Replication of the write aliases is skipped, however, for reason explained in the PR, but this is not explained in the [docs](https://www.elastic.co/guide/en/elasticsearch/reference/current/xpack-ccr.html#ccr-update-leader-index) and could lead to deployment design misunderstandings.