[9.0][DOCS] Renames ML get usage info APIs back to get stats

[szabosteve]

Backport of #4299

[github-actions]

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

API	Status	Request	Response
ml.get_data_frame_analytics_stats	🟢	12/12	12/12
ml.get_datafeed_stats	🟢	27/27	27/27
ml.get_job_stats	🟢	32/32	32/32

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

[github-actions]

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

API	Status	Request	Response
ml.get_data_frame_analytics_stats	🟢	12/12	12/12
ml.get_datafeed_stats	🟢	27/27	27/27
ml.get_job_stats	🟢	32/32	32/32

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