The information for this specific case isn't anywhere. The best that you can do is understand what triggers each of the events, then ask yourself which of those triggers are pertinent to your current issue.
The above link contains a great deal of information regarding how to use scripts within Livecycle Designer. The section that you would be most interested in in order to learn about the available events can be found under "Events >> List of Events".
You needed to use the initialize event so that the text field would be updated when the document was opened. This ensure that the text field has the value of the default radio button when it is opened for the first time.
You needed to use the change event so that, when the use selects a different radio button, the text field is updated with the new value.
That's about it. That PDF file that I link to above is a bit long, but it's a great place to get a good grasp of the basics of scripts.
If you have any further questions, feel free to ask. 😃 Have a good one.