Clarify collection contracts and migration docs

namespaces.go

@@ -20,8 +20,9 @@ type Namespaces struct {
 // defaults on the new view and leaves the receiver unchanged; resource
 // existence is checked when selected. An empty field clears that resource's
 // scope. Captured configuration references, defaults, and global configuration
-// inspection are not rewritten. Invalid namespace syntax or a nil engine
-// returns an error matching ErrInvalidConfig.
+// inspection are not rewritten. [Engine.ListNamespaces] and
+// [Engine.ListCollections] remain global inventories. Invalid namespace syntax
+// or a nil engine returns an error matching ErrInvalidConfig.
 func (e *Engine) WithNamespaces(namespaces Namespaces) (*Engine, error) {
 	if e == nil || e.internal == nil {
 		return nil, fmt.Errorf("%w: engine is nil", ErrInvalidConfig)