core: Improve RemoveBoxInfo and WriteBoxInfo docs a bit

lib/vagrant/action/builtin/remove_box_info.rb

@@ -3,7 +3,8 @@ require "log4r"
 module Vagrant
   module Action
     module Builtin
-      # This middleware will persist some extra information about the base box
+      # This middleware will remove additional information about the base box
+      # from state file
       class RemoveBoxInfo
         def initialize(app, env)
           @app    = app

lib/vagrant/action/builtin/write_box_info.rb

@@ -4,6 +4,7 @@ module Vagrant
   module Action
     module Builtin
       # This middleware will persist some extra information about the base box
+      # on a state file
       class WriteBoxInfo
         def initialize(app, env)
           @app    = app