[DOCS] Add ML rare functions (elastic/x-pack-elasticsearch#1351)
[DOCS] Add ML rare functions (elastic/x-pack-elasticsearch#1351)

* [DOCS] Add ML rare functions

* [DOCS] Address feedback in ML rare functions

Original commit: elastic/x-pack-elasticsearch@388274557ce30865145fbbb1d859147ca9e0f974

gfyoung/elasticsearch,uschindler/elasticsearch,uschindler/elasticsearch,strapdata/elassandra,nknize/elasticsearch,strapdata/elassandra,GlenRSmith/elasticsearch,gingerwizard/elasticsearch,strapdata/elassandra,HonzaKral/elasticsearch,gingerwizard/elasticsearch,vroyer/elassandra,robin13/elasticsearch,vroyer/elassandra,scorpionvicky/elasticsearch,nknize/elasticsearch,robin13/elasticsearch,coding0011/elasticsearch,nknize/elasticsearch,scorpionvicky/elasticsearch,uschindler/elasticsearch,gfyoung/elasticsearch,uschindler/elasticsearch,gfyoung/elasticsearch,HonzaKral/elasticsearch,robin13/elasticsearch,strapdata/elassandra,GlenRSmith/elasticsearch,robin13/elasticsearch,nknize/elasticsearch,gingerwizard/elasticsearch,gingerwizard/elasticsearch,gfyoung/elasticsearch,GlenRSmith/elasticsearch,scorpionvicky/elasticsearch,gingerwizard/elasticsearch,GlenRSmith/elasticsearch,nknize/elasticsearch,vroyer/elassandra,scorpionvicky/elasticsearch,gingerwizard/elasticsearch,HonzaKral/elasticsearch,strapdata/elassandra,coding0011/elasticsearch,robin13/elasticsearch,gfyoung/elasticsearch,coding0011/elasticsearch,uschindler/elasticsearch,scorpionvicky/elasticsearch,GlenRSmith/elasticsearch,coding0011/elasticsearch,coding0011/elasticsearch,gingerwizard/elasticsearch,HonzaKral/elasticsearch