Add info on client-side button actions.

ui/button_actions.md

@@ -1,6 +1,14 @@
 ### Button Actions
 
-When a toolbar button is pressed current controller's `button` or `x_button` method is called. This is tru with the exception of some client-side only actions.
+Toolbar button actions can be server-side or client-side.
+
+Currently most buttons are server-side but the preferred way of writing new
+code is calling the API from client-side button actions.
+
+#### Server-side Button Actions
+
+When a toolbar button is pressed current controller's `button` or `x_button`
+method is called.
 
 The identifier of the button that was pressed is passed in `params[:pressed]`.
 
@@ -12,6 +20,14 @@ The `x_button` or `button` handlers should follow this pattern:
  end
 ```
 
-Meaning that just a lookup in the list of valid actions for the controller is done and the action is called if found. Nothing else.
+Meaning that just a lookup in the list of valid actions for the controller is
+done and the action is called if found. Nothing else.
+
+Cleaning up the handlers to this form will allow us to do furter refactorings
+and make the button actions plugable.
+
+#### Client-side Button Actions
+In [Toolbars and Buttons](toolbars.md#javascript-only-buttons) you can see how to define javascript-only buttons.
 
-Cleaning up the handlers to this form will allow us to do furter refactorings and make the button actions plugable.
+Such button can perform changes in the UI or it can call the ManageIQ REST API
+to get work done. See [Calling the API](calling_api.md).