[ruby/reline] Add a comment for VK_MENU

https://github.com/ruby/reline/commit/ee307c2b01

lib/reline/windows.rb

@@ -95,7 +95,7 @@ class Reline::Windows
   end
 
   VK_RETURN = 0x0D
-  VK_MENU = 0x12
+  VK_MENU = 0x12 # ALT key
   VK_LMENU = 0xA4
   VK_CONTROL = 0x11
   VK_SHIFT = 0x10