Add footnote about the indexing performance differences (#3758)

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).

[Preview of the
doc](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/3758/deploy-manage/deploy/elastic-cloud/differences-from-other-elasticsearch-offerings#elasticsearch)

The footnote: 
<img width="878" height="72" alt="image"
src="https://github.com/user-attachments/assets/b1a750f8-3669-45cf-bc54-da1052ecd8be"
/>
<img width="876" height="157" alt="image"
src="https://github.com/user-attachments/assets/1442c0b6-9287-45c1-b447-8a96aefc3297"
/>

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 baseline write latency in {{serverless-short}} is 200ms [^1^](#footnote-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 +103,8 @@ 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^ $$$footnote-1$$$ In {{serverless-short}}, Elastic ensures data durability by storing indexed data in an [object store](https://www.elastic.co/blog/elastic-serverless-architecture) rather than local replicas. Writes are batched over a 200ms window to ensure durability while optimizing performance and cost, 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. Because of a higher baseline write latency, {{serverless-short}} indexing can be scaled by increasing concurrent indexing clients.
+
 ### 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).