container: improve documentation

This change removes inconsistencies collected over time in this package.

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

container/path.go

@@ -15,7 +15,10 @@ import (
 
 const (
 	// Nonexistent is a path that cannot exist.
-	// /proc is chosen because a system with covered /proc is unsupported by this package.
+	//
+	// This path can never be presented by the kernel if proc is mounted on
+	// /proc/. This can only exist if parts of /proc/ is covered, or proc is not
+	// mounted at all. Neither configuration is supported by this package.
 	Nonexistent = fhs.Proc + "nonexistent"
 
 	hostPath    = fhs.Root + hostDir