Clarify index settings & APIs that are not available in serverless (#1728)

## Description

There are some APIs and index settings that are not available in
serverless mode. Using such API or index settings will get such
response.

Two main things:

### [1]

It's not clear in docs that explain "why" these settings are designed as
not available.

Reason per we checked internally is that, _"These are defensive settings
meant to protect the index from misuse that may lead to cluster
instability so I don't believe we can expose these in Serverless. Since
Elastic has responsibility for supporting cluster stability we have to
make decisions about what you can and can not do to destabilize a
cluster."_

We'd like to make it clear in public too. 

Also, some other pointers taken into consideration:

- Avoid mentioning "misuse" since that sounds like user's fault
- Emphasize it's Elastic's responsibility to keep the cluster stability.
- Cover also cluster level and node level setting description that fully
managed by Elastic


### [2]

There's a detailed error message when using not available API. But
there's no such error message for not available index settings. It's
logically unbalanced.

Either we should remove the message from unavailable API, or we add the
error message for unavailable index settings.

IMHO, at this time point, I'd like to add the error message for
unavailable index settings, because there must have been a reason that
we decided to put "unavailable API error message" into the doc.

In the long run, I'd think we should remove both. 

### Side notes

Note the difference between index settings, and node/cluster settings. 
Per the [existing
doc](https://www.elastic.co/docs/deploy-manage/deploy/elastic-cloud/differences-from-other-elasticsearch-offerings),
we don't allow any node settings and cluster settings, they are fully
managed by Elastic.
We only allow limited index settings. 

I tried to make this point clear in my doc PR too. 

## PR Preview

API

-
https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/1728/deploy-manage/deploy/elastic-cloud/differences-from-other-elasticsearch-offerings#elasticsearch-differences-serverless-apis-availability

Index settings

-
https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/1728/deploy-manage/deploy/elastic-cloud/differences-from-other-elasticsearch-offerings#elasticsearch-differences-serverless-settings-availability

---

cc @ppf2 @jakommo @maggieghamry as we discussed together
cc @dbrimley @leemthompo

---------

Co-authored-by: Edu González de la Herrán <[email protected]>

Author: kunisen

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

@@ -162,7 +162,10 @@ These recommendations do not apply to indices using better binary quantization (
 
 ## Available {{es}} APIs [elasticsearch-differences-serverless-apis-availability]
 
-Because {{serverless-full}} manages infrastructure automatically, certain Elasticsearch APIs are not available:
+In {{serverless-full}}, access is limited to a subset of {{es}} APIs, as Elastic manages the underlying infrastructure. These restrictions help maintain cluster stability, availability, and data integrity, ensuring reliable operation of Serverless projects.
+
+The following {{es}} APIs are not available in {{serverless-full}}:
+
 
 Infrastructure operations
 :   * All `_nodes/*` operations
@@ -201,7 +204,7 @@ Refer to the [{{es-serverless}} API reference](https://www.elastic.co/docs/api/d
 
 ## Available {{es}} settings [elasticsearch-differences-serverless-settings-availability]
 
-In {{serverless-full}} Elasticsearch projects, you can only configure [index-level settings](elasticsearch://reference/elasticsearch/index-settings/index.md). Cluster-level settings and node-level settings are fully managed by Elastic.
+In {{serverless-full}} projects, configuration available to users is limited to certain [index-level settings](elasticsearch://reference/elasticsearch/index-settings/index.md), while Elastic manages cluster-level and node-level settings to maintain stability, availability, performance, and data integrity. These restrictions help ensure the reliability of Serverless projects.
 
 Available settings
 :   **Index-level settings**: Settings that control how documents are processed, stored, and searched are available to end users. These include:
@@ -219,6 +222,24 @@ Managed settings
     * Shard allocation
     * Resource management
 
+When attempting to use an unavailable index setting, you'll receive this error:
+
+```json
+{
+    "error": {
+        "root_cause": [
+            {
+                "type": "illegal_argument_exception",
+                "reason": "Settings [xyz] are not available when running in serverless mode"
+            }
+        ],
+        "type": "illegal_argument_exception",
+        "reason": "Settings [xyz] are not available when running in serverless mode"
+    },
+    "status": 400
+}
+```
+
 ## Learn more
 
 - [{{serverless-full}} roadmap](https://www.elastic.co/cloud/serverless/roadmap): See upcoming features and development plans for the Serverless platform