Update docs about event in the plugin

[steinkel]

Update event names and usage

[steinkel]

I'd improve the documentation to add the following items to all the events:

• Name (as we have now)
• Why: Describe the reason for this event, add typical use cases for the event like "Useful to validate user data and update it before saving it"
• When is it triggered: Describe when the plugin is dispatching the event
• Preconditions: Describe any specific data passed in the event data
• Postconditions: Describe what happens if the event is stopped, of if anything is expected in the event data

[ajibarra]

@steinkel is this still valid?