ECH upgrades with API and upgrade assistant

For ECH users using the API we might want to provide instructions for upgrades.

In such case we need to include details with the upgrade assistant usage when doing major upgrades

This has been highlighted in [this comment](https://github.com/elastic/docs-content/pull/3398#issuecomment-3409946933).

> Could we add a note that if user use API way to upgrade on ECH, they need to run GET /_migration related APIs to get their clusters ready to upgrade please?
　
> This API set is called by Kibana upgrade assistant, but if user use terraform or other ways to upgrade from cloud, they can bypass upgrade assistant, and cause troubles.
https://www.elastic.co/docs/api/doc/elasticsearch/group/endpoint-migration
　
> FWIW, we have [an internal ticket](https://elasticco.atlassian.net/browse/CP-10886) for this.

## Action
added by (@georgewallace)

- Review the existing [API specification for Feature Upgrade](https://github.com/elastic/elasticsearch-specification/blob/main/specification/migration/post_feature_upgrade/PostFeatureUpgradeRequest.ts#L30). It clearly states a TIP that states `The API is designed for indirect use by the Upgrade Assistant. We strongly recommend you use the Upgrade Assistant.`
- Update the description in v8, v9, main with updated text in the `PostFeatureUpgradeRequest.ts` file.
- Ask the API docs release team to release an update to the docs.