[DOC] match_only_text handler

[macohen]

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 and all versions that are affected.
match_only_text handler was originally requested by someone using Elastic Common Schema which has a dependency on this feature. Subsequently, we realized this could be a useful performance enhancement and are targeting a 2.12 release.

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
opensearch-project/OpenSearch#6836

cc: @rishabhmaurya (the developer working on this)

[hdhalter]

Thanks for entering the issue, @macohen ! @rishabhmaurya , will you be creating the PR for this doc update?

[hdhalter]

Assigning @kolchfa-aws for visibility.