Elasticsearch version (bin/elasticsearch --version):

6.1 -> master

Description of the problem including expected versus actual behavior:

• The get data feeds API has a allow_no_datafeeds query parameter per https://github.com/elastic/elasticsearch/blob/master/x-pack/plugin/src/test/resources/rest-api-spec/api/ml.get_datafeeds.json It is not mentioned in https://www.elastic.co/guide/en/elasticsearch/reference/master/ml-get-datafeed.html
• Ditto https://github.com/elastic/elasticsearch/blob/master/x-pack/plugin/src/test/resources/rest-api-spec/api/ml.get_datafeed_stats.json
• Ditto https://github.com/elastic/elasticsearch/blob/master/x-pack/plugin/src/test/resources/rest-api-spec/api/ml.stop_datafeed.json

(#44499 will fix the issues above. - Istv)

• The get jobs API has an allow_no_jobs query parameter per https://github.com/elastic/elasticsearch/blob/master/x-pack/plugin/src/test/resources/rest-api-spec/api/ml.get_jobs.json
• Ditto https://github.com/elastic/elasticsearch/blob/master/x-pack/plugin/src/test/resources/rest-api-spec/api/ml.get_job_stats.json
• Ditto https://github.com/elastic/elasticsearch/blob/master/x-pack/plugin/src/test/resources/rest-api-spec/api/ml.close_job.json

There is also a minor issue in https://www.elastic.co/guide/en/elasticsearch/reference/master/get-dfanalytics.html where the allow_no_match appears in the wrong section. (#44498 will take care of this. – Istv)

Steps to reproduce:
N/A

Provide logs (if relevant):
N/A