* [DOCS] Add details about ML count functions
* [DOCS] Address feedback in ML count functions
* [DOCS] Clarify ML population analysis in non-zero count functions
Original commit: elastic/x-pack-elasticsearch@24dbeba891
* [DOCS] Add ML analytical functions
* [DOCS] Add pages for ML analytical functions
* [DOCS] Add links to ML functions from API definitions
Original commit: elastic/x-pack-elasticsearch@ae50b431d3