name twigils for private attributes

Author: gfldex

doc/Language/typesystem.pod6

@@ -28,8 +28,9 @@ TODO
 
 =head4 Private Attributes
 
-Private attributes do not have public accessor methods generated automatically.
-As such they can not be altered from outside the class they are defined in.
+Private attributes are addressed with any of the twigils C<$!>, C<@!> and
+C<%!>. They do not have public accessor methods generated automatically.  As
+such they can not be altered from outside the class they are defined in.
 
     class C {
         has $!priv;