container: improve documentation

This change removes inconsistencies collected over time in this package.

Signed-off-by: Ophestra <cat@gensokyo.uk>

container/vfs/unfold.go

@@ -6,6 +6,7 @@ import (
 	"strings"
 )
 
+// UnfoldTargetError is a pathname that never appeared in a mount hierarchy.
 type UnfoldTargetError string
 
 func (e UnfoldTargetError) Error() string {
@@ -27,6 +28,7 @@ func (n *MountInfoNode) Collective() iter.Seq[*MountInfoNode] {
 	return func(yield func(*MountInfoNode) bool) { n.visit(yield) }
 }
 
+// visit recursively visits all visible mountinfo nodes.
 func (n *MountInfoNode) visit(yield func(*MountInfoNode) bool) bool {
 	if !n.Covered && !yield(n) {
 		return false