doc is default on Attribute

Author: gfldex

doc/Type/Attribute.pod6

@@ -30,6 +30,22 @@ The usual way to obtain an object of type C<Attribute> is by introspection:
     $a.set_value($instance, [1, 2, 3]);
     say $a.get_value($instance);        # [1 2 3]
 
+=head1 Traits
+
+=head2 X<Trait is default|trait,is default (Attribute)>
+
+An attribute that is assinged L<Nil|/type/Nil> will revert to its default value
+set with the trait C<is default>.
+
+    class C {
+        has $.a is default(42) is rw = 666
+    }
+    my $c = C.new;
+    say $c;
+    $c.a = Nil;
+    say $c;
+    # OUTPUT«C.new(a => 666)␤C.new(a => 42)␤»
+
 =head1 Methods
 
 =head2 method name