[DOC] Update k-NN plugin documentation to reflect explain API for Exact/ANN/Radial/Disk/Filtering k-nn search.

[neetikasinghal]

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.
With recent changes the end users of knn plugin should be able to use explain API for Exact/ANN/Radial/Disk/Filtering based k-nn search which can help them debug and troubleshoot issues on their own.

Version: 3.0 & +

What other resources are available?
PR: opensearch-project/k-NN#2403
Proposal for explain is given here: opensearch-project/k-NN#875 (comment)
Output : opensearch-project/k-NN#875 (comment)