[DOC] document the nested_path parameter in agent search tools

[zhichao-aws]

What do you want to do?
Per customer request, in 2.16 we will support nested field in agent search tools (neural sparse tool, vectorDB tool, RAG Tool). We add a new parameter to these tools, and we need to document this change

• 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.

*Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.
2.16
What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
opensearch-project/skills#350

[zhichao-aws]

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