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.
Generally, "failure" indicates an expected state of things (e.g. a CI test didn't pass), while "Error" is an unexpected event (Deployment failed because server was unresponsive). TO the hub it doesn't make any difference, so users are free to use them distinctively or not for their own information.