Add descriptions for migrate, modify, and data stream stats

[lcawl]

Relates to #964

This PR adds descriptions from https://www.elastic.co/guide/en/elasticsearch/reference/master/data-stream-stats-api.html, https://www.elastic.co/guide/en/elasticsearch/reference/master/indices-migrate-to-data-stream.html, and https://www.elastic.co/guide/en/elasticsearch/reference/master/modify-data-streams-api.html

[github-actions]

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

API	Status	Request	Response
indices.data_streams_stats	🟢	4/4	4/4
indices.migrate_to_data_stream	⚪	Missing test	Missing test
indices.modify_data_stream	⚪	Missing test	Missing test

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

[abdonpijpelink]

LGTM