Document source-root configuration controls

layered.go

@@ -5,7 +5,8 @@ import "go.maximumdirect.net/configkit/internal/engine"
 // Input selects one source, its application destination, whether its imports are
 // enabled, and any explicit filesystem context. A zero Source is invalid; use
 // NoSource() for an omitted main input. Sources keep their documented reuse and
-// reader ownership rules.
+// reader ownership rules. Each source's root controls are interpreted before
+// placement; a destination named config remains ordinary application data.
 type Input struct {
 	Source Source
 	// Destination establishes the mapping path for Source. Imports inherit this