[DOC] Documentation for archived cluster and index settings.

[ankitkala]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request. Provide a summary of the request and all versions that are affected.
Whenever OpenSearch stumbles upon a cluster/index setting which it doesn't recognize, it moves the setting to archived settings. Customer can't modify any other setting unless these archived settings are cleared up.

We don't have any documentation around this behaviour and how to remediate.

Archived cluster level settings can be cleared up by

PUT _cluster/settings
{
  "persistent": {
    "archived.*": null
  }
}

Removal for archived index setting doesn't work as of now(opensearch-project/OpenSearch#2803), we are introducing the fix here which allows users to clear the settings on closed indices.
To clear the archived setting on an index,

1. Close the index.
2. Clear the setting.
3. Reopen the index.

POST /my-index/_close

PUT /my-index/_settings
{
  "archived.*": null
}

POST /my-index/_open

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.

• Issue for bug: Delete deprecated index settings "archived.*" OpenSearch#2803
• PR for fix: [BUG] Add support to clear archived index settings OpenSearch#9019
• Sample reference: https://www.elastic.co/guide/en/elasticsearch/reference/current/archived-settings.html (there is a deviation here through. We only allow removing archived settings on closed indices).

[hdhalter]

Thanks for entering this issue, @ankitkala ! Can you please open a PR to update the documentation? Thank you.