[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)

[opensearch-ci-bot]

Hi @neetikasinghal
,

As part of the entrance criteria, all the documentation pull requests need to be drafted and in technical review.

Since there is no pull request linked to this issue, please take one of the following actions:

• Create the pull request and link it to this issue.
  
• If you already have a pull request created, please link it to this issue.
  
• If this feature is not targeted for the currently labeled release version, please update the issue with the correct release version.
  

Please note: Missing documentation can block the release and cause delays in the overall process.

Thank you!