Streams: Document managed Elasticsearch assets

[flash1293]

https://elastic.slack.com/archives/C081J8HAZ50/p1761214300222479

When managing classic or wired streams through the streams UI or API, then Elasticsearch-level assets are created. These are considered managed and shouldn't be changed directly through the Elasticsearch-level APIs. If you manage a stream through the streams API/UI, then you should stick with this where possible.

It's still completely OK to change non-managed ingest pipelines, templates and so on even if using streams, just the assets marked as "managed" and the per-datastream mappings and settings should not be touched. This works similar to integration-managed Elasticsearch assets.