[DOCS] Descriptions for SQL APIs

[abdonpijpelink]

Relates to #964.

Adds descriptions for SQL APIs. Descriptions were sourced from https://www.elastic.co/guide/en/elasticsearch/reference/current/sql-apis.html

[szabosteve]

One small suggestion, otherwise LGTM!

[github-actions]

Following you can find the validation results for the APIs you have changed.

API	Status	Request	Response
sql.clear_cursor	🟢	1/1	1/1
sql.delete_async	⚪	Missing test	Missing test
sql.get_async_status	⚪	Missing test	Missing test
sql.get_async	⚪	Missing test	Missing test
sql.query	🟢	5/5	5/5
sql.translate	🟢	1/1	1/1

You can validate these APIs yourself by using the make validate target.