[DOCS] Update ML open job API (elastic/x-pack-elasticsearch#1875)
[DOCS] Update ML open job API (elastic/x-pack-elasticsearch#1875)

Original commit: elastic/x-pack-elasticsearch@00a76d79ae6f90e9d112fb2043f07fad1b585c5f

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