Add footnote about the indexing performance differences between Serverless and ECH

Adding more information to explain why single-document indexing in Serverless may appear to be slower than in an ECH environment.

Fixes [#385](elastic/docs-content-internal#385).

Author: yetanothertw

deploy-manage/deploy/elastic-cloud/differences-from-other-elasticsearch-offerings.md

@@ -86,7 +86,7 @@ This table compares Elasticsearch capabilities between {{ech}} deployments and S
 | [**AI Assistant**](/solutions/observability/observability-ai-assistant.md) | ✅ | ✅ | |
 | **Behavioral analytics** | ❌ (deprecated in 9.0) | ❌ | Not available in Serverless |
 | [**Clone index API**](https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-indices-clone) | ✅ | **Planned** | Anticipated in a future release |
-| [**Bulk indexing**](/deploy-manage/production-guidance/optimize-performance/indexing-speed.md#_use_bulk_requests) |  ✅ | ✅ | The maximum bulk request response time in {{serverless-short}} is 200ms |
+| [**Bulk indexing**](/deploy-manage/production-guidance/optimize-performance/indexing-speed.md#_use_bulk_requests) |  ✅ | ✅ | The maximum bulk request response time in {{serverless-short}} is 200ms ^1^ |
 | [**Cross-cluster replication**](/deploy-manage/tools/cross-cluster-replication.md) | ✅ | **Planned** | Anticipated in a future release |
 | [**Cross-cluster search**](/solutions/search/cross-cluster-search.md) | ✅ | **Planned** | Anticipated in a future release |
 | **Data lifecycle management** | - [ILM](/manage-data/lifecycle/index-lifecycle-management.md) <br>- [Data stream lifecycle](/manage-data/lifecycle/data-stream.md) | [Data stream lifecycle](/manage-data/lifecycle/data-stream.md) only | - No data tiers in Serverless <br>- Optimized for common lifecycle management needs |
@@ -103,6 +103,9 @@ This table compares Elasticsearch capabilities between {{ech}} deployments and S
 | [**Watcher**](/explore-analyze/alerts-cases/watcher.md) | ✅ | ❌ | Use **Kibana Alerts** instead, which provides rich integrations across use cases |
 | **Web crawler** | ❌ (Managed Elastic Crawler discontinued with Enterprise Search in 9.0) | Self-managed only | Use [**self-managed crawler**](https://github.com/elastic/crawler) |
 
+
+^1^ In {{serverless-short}}, data durability is achieved through an object store rather than local replicas. Writes are grouped into batches of 200ms to ensure consistency and persistence, which means that single-document indexing can appear slower than in {{ech}}. However, this design makes {{serverless-short}} more scalable and resilient to high indexing loads without relying on in-cluster replication for fault tolerance.
+
 ### Observability
 
 This table compares Observability capabilities between {{ech}} deployments and Observability Complete Serverless projects. For more information on Observability Logs Essentials Serverless projects, refer to [Observability feature tiers](../../../solutions/observability/observability-serverless-feature-tiers.md).