Document iconClass and displayName fields for custom resource action discovery

[crenshaw-dev]

Summary

Document the iconClass and displayName fields which can be set via custom resource action discovery scripts.

local actions = {}
actions["create-job"] = {
  ["iconClass"] = "fa fa-fw fa-play",
  ["displayName"] = "Create Job"
}
return actions

Motivation

It's currently a hidden feature.

Proposal

Add info to the custom resource actions docs page.

[sikehish]

Can I be assigned this issue?