[DOC]Multi innerhit in nested k-NN field

[heemin32]

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.
Support of multiple inner hits for nested k-NN field

Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.
2.19

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
opensearch-project/k-NN#2249
opensearch-project/k-NN#2283