[DOCS] Add cloud trial link to reindex API description (#6181)

(cherry picked from commit 42814cd)

Author: charlotte-hoblik

specification/_global/reindex/ReindexRequest.ts

@@ -50,7 +50,8 @@ import { Destination, Source } from './types'
  * * To automatically create a data stream or index with a reindex API request, you must have the `auto_configure`, `create_index`, or `manage` index privilege for the destination data stream, index, or alias.
  * * If reindexing from a remote cluster, the `source.remote.user` must have the `monitor` cluster privilege and the `read` index privilege for the source data stream, index, or alias.
  *
- * If reindexing from a remote cluster, you must explicitly allow the remote host in the `reindex.remote.whitelist` setting.
+ * If reindexing from a remote cluster into a cluster using Elastic Stack, you must explicitly allow the remote host using the `reindex.remote.whitelist` node setting on the destination cluster.
+ * If reindexing from a remote cluster into an Elastic Cloud Serverless project, only remote hosts from [Elastic Cloud Hosted](https://cloud.elastic.co/registration?page=docs&placement=docs-body) are allowed.
  * Automatic data stream creation requires a matching index template with data stream enabled.
  *
  * The `dest` element can be configured like the index API to control optimistic concurrency control.