[DOC] Documentation for GRPC GA in 3.2 release

[karenyrx]

What do you want to do?
Update the documentation for GRPC API

• 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.
GRPC APIs are currently marked as an experimental feature. In the upcoming 3.2 OS release, we plan to make certain GRPC APIs generally available. This issue captures the changes that need to be made. Changes will specifically be for the Bulk and KNN Search APIs. Limitations will be documented, and some links may be updated.

Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.
3.2

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
https://docs.opensearch.org/latest/api-reference/grpc-apis/index/