Document Chef Server's REST interface to the ACL subsystem

[sean-horn]

This location should probably have full documentation about the ACL subsystem. https://docs.chef.io/api_chef_server.html

Currently the only documentation for those functions is in https://github.com/chef/knife-acl

This explains at a very high level how the ACL system works https://github.com/chef/chef-server/blob/master/doc/ChefServerPermissions_v1.3.pdf