[DOC] Update documentation around keyword fields

[harshavamsi]

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.
In opensearch-project/OpenSearch#7057 we introduced support for keyword fields and doc values. We would need to update documentation to correct that doc values are enabled by default for keyword fields.

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.