[DOC] Force-merge API supports primary_only parameter

[gaobinlong]

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• Request a change to existing documentation
• Add new documentation
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Tell us about your request. Provide a summary of the request and all versions that are affected.
In release 2.13.0, we'll introduce the parameter primary_only for force-merge API, we need to document it.

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
opensearch-project/OpenSearch#11269