[Console] Added a little explaination about Command#interact()

src/Symfony/Component/Console/Command/Command.php

@@ -175,6 +175,10 @@ class Command
     /**
      * Interacts with the user.
      *
+     * This method is executed before the InputDefinition is validated.
+     * This means that this is the only place where the command can
+     * interactively ask for values of missing required arguments.
+     *
      * @param InputInterface  $input  An InputInterface instance
      * @param OutputInterface $output An OutputInterface instance
      */