[API docs] Add descriptions to API tags

[szabosteve]

Overview

We use tags to group APIs by feature. Tags should be descriptions to be more understandable.
The existing tags are available in the elasticsearch-shared-overlays.yaml file.
Recently, a new REST API page was created in the Elasticsearch reference docs available here. The page contains a description for every API namespace. Leverage these descriptions, make them shorter when possible, and add them to the tag descriptions. Create descriptions for tags that don't have a corresponding API namespace description on the REST API page.

Resources

• Example of a tag description (CAT)
• Description of API endpoints on the REST API page