Clarify ECH node bootlooping documentation

[kunisen]

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

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

[AlexP-Elastic]

LGTM

[maggieghamry]

LGTM! small note: perhaps having an example next to "via a client tool or API" (for example terraform-ec) or perhaps utilize a knowledge article separately.

[kunisen]

Thanks all for the quick review! 🙏

Also thanks @maggieghamry sure, I added terraform ec provider as an example as suggested.