The docs list actions for es 5.x. Does that same list apply to 6.x?
A new action is generally created when a new feature in ES is added, so that list is still a very valid starting point. I wrote a program to scan ES code and generate that list, I don’t think I have it anymore 
The good news is that ReadonlyREST logs print an “ACT” field when requests are received, so you can get a feeling of what actions are being triggered/used by the clients.