Clarify ECH node bootlooping documentation (#1093)

## Description

Per an internal discussion (`sdh-cp #9409`), we would like to update the
ECH doc to clarify the node bootlooping related doc.

### Background

A setting is configured in Elasticsearch but it reports error

```bash
org.elasticsearch.common.settings.SettingsException: 
The configuration setting [xpack.security.authc.realms.foobar.foobar1.foobar2.client_secret] is required
```

And during the troubleshooting, we found the setting is configured via
client tooling and it's not properly configured (zero byte).

Based on that, we logically thought to solve the issue, users should 
- understand this may happen if the keystore setting
(`[xpack.security.authc.realms.foobar.foobar1.foobar2.client_secret]` to
be configured on ECH side) is invalid, missing or improperly configured
- understand the way to solve is to check if the keystore is valid or
not and make sure it's configured properly
- additionally if users configured this via client tooling, they should
try configuring this in cloud UI directly as a way to isolate the cause

### After PR being merged

The following orange section will be added


![image](https://github.com/user-attachments/assets/11210aa0-00d0-45ad-8e0c-2d6a15fb6b60)


![image](https://github.com/user-attachments/assets/bfc67644-a79e-465e-b4af-7a85d4650548)

Author: kunisen

troubleshoot/monitoring/node-bootlooping.md

@@ -49,8 +49,16 @@ If you’re unable to remediate the failing plan’s root cause, you can attempt
 The most frequent cause of a failed deployment configuration change is due to invalid or mislocated [secure settings](/deploy-manage/security/secure-settings.md). This can frequently be discovered by searching {{es}} logs for one of the following error messages:
 
 ```sh
+# Typical Error Message 
+#------------------------------
+[ERROR][org.elasticsearch.bootstrap.Elasticsearch] ... fatal exception while booting Elasticsearch
 IllegalStateException: security initialization failed
+
 java.lang.IllegalArgumentException: unknown secure setting
+
+org.elasticsearch.common.settings.SettingsException: 
+The configuration setting [xpack.security.authc.realms.foobar.foobar1.foobar2.client_secret] is required
+#------------------------------
 ```
 
 These are settings typically added to the keystore for the purpose of:
@@ -62,7 +70,6 @@ The keystore allows you to safely store sensitive settings, such as passwords, a
 
 The following sections detail some secure settings problems that can result in a configuration change error that can prevent a deployment from restarting. You might diagnose these plan failures via the logs or via their [related exit codes](/deploy-manage/maintenance/start-stop-services/start-stop-elasticsearch.md#fatal-errors) `1`, `3`, and `78`.
 
-
 ### Invalid or outdated values [ec-config-change-errors-old-values]
 
 The keystore does not validate any settings that you add, so invalid or outdated values are a common source of errors when you apply a configuration change to a deployment.
@@ -91,6 +98,13 @@ When you configure third-party authentication, it’s important that all require
 
 In some cases, settings may accidentally be added to the keystore that should have been added to the [{{es}} user settings file](/deploy-manage/deploy/elastic-cloud/edit-stack-settings.md). It’s always a good idea to check the [lists of reloadable settings](/deploy-manage/security/secure-settings.md) to determine if a setting can be stored in the keystore. Settings that can safely be added to the keystore are flagged as `Secure`.
 
+### Missing or improperly configured
+
+The error message `The configuration setting [...] is required` indicates that the corresponding setting is configured and present in the Elasticsearch instance via [Elasticsearch user settings](/deploy-manage/deploy/elastic-cloud/edit-stack-settings.md#ec-add-user-settings), but is either missing or improperly configured in [secure settings](/deploy-manage/security/secure-settings.md). Please review your [secure settings](/deploy-manage/security/secure-settings.md) to ensure they are configured correctly.
+
+Additionally, if you configure these settings via a client tool, such as the [Terraform Provider for Elastic Cloud](https://github.com/elastic/terraform-provider-ec), or through an API and encounter the error, try configuring the settings directly in the Cloud UI to isolate the cause. If configuring in the Cloud UI does not result in the same error, it suggests that the keystore setting is valid, and the method of configuration should be examined. Conversely, if the same error is reported, it suggests that the keystore setting may be invalid and should be reviewed.
+
+
 
 ## Expired custom plugins or bundles [ec-config-change-errors-expired-bundle-extension]