[DOCS] Adds description to the Search Application APIs

[szabosteve]

Overview

Related to #964.

Adds descriptions to Search Application API properties. Source of info:

• https://www.elastic.co/guide/en/elasticsearch/reference/current/search-application-apis.html

[github-actions]

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

API	Status	Request	Response
search_application.delete_behavioral_analytics	⚪	Missing test	Missing test
search_application.delete	⚪	Missing test	Missing test
search_application.get_behavioral_analytics	⚪	Missing test	Missing test
search_application.get	⚪	Missing test	Missing test
search_application.list	⚪	Missing test	Missing test
search_application.post_behavioral_analytics_event	🟠	Missing type	Missing type
search_application.put_behavioral_analytics	⚪	Missing test	Missing test
search_application.put	⚪	Missing test	Missing test
search_application.render_query	🟠	Missing type	Missing type
search_application.search	⚪	Missing test	Missing test

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

[abdonpijpelink]

LGTM