Clear up Addon Layer usage, remove partially wrong debugging explanation

Cleared up Layer usage, remove the confusing and partially wrong explanatio of the DefaultBrowserLayer.

Added an example about layer subclassing referencing the original issue that just got removed.

Maybe @ miohtama wants to have a look because of: collective/collective.developermanual@f2edae2e7788d8e95802adf217485ad46c0bd364