Continues the work started for #5917 (already covered Targets, Layouts,
LayoutRenderers and Target Wrappers), extending the same pattern to the
NLog.Filters namespace so default values show up in IntelliSense and the
generated API docs.
Adds a regression test that scans all settable NLog.Filters properties
against the generated XML documentation file and fails if any of them is
missing a documented default value.
