Expanded documentation for Webhooks 2.0

Docs people: @fredlf @SvenDowideit 
Hub people: @kencochrane @fxdgear

Please review! Also note that this feature isn't in production yet but is slated for release in a few days.
Again, left to the user's discretion =/ @kencochrane might have information on the original intent, but it has no technical purpose other than being retrieved in the history logs.