Partially document the 'is built' trait

doc/Type/Attribute.pod6

@@ -111,6 +111,29 @@ default accessor for the attribute will return a writable value.
    CATCH { default { put .^name, ': ', .Str } };
    # OUTPUT: «X::Assignment::RO: Cannot modify an immutable Any␤»
 
+=head2 X<trait is built|trait,is built (Attribute)>
+
+Defined as:
+
+    multi sub trait_mod:<is>(Attribute:D $a, :$built!)
+
+By default, this trait allows setting up a I«private attribute» during object
+construction via C«.new». The same trait can be used to prevent setting up a
+I«public attribute» via C«.new» by passing it the boolean value C«False».
+
+    class Foo {
+        has $!bar is built; # same as `is built(True)`
+        has $.baz is built(False);
+
+        method bar {
+            $!bar
+        }
+    }
+
+    my $foo = Foo.new(bar => 1, baz => 2);
+    say $foo.bar; # «1␤»
+    say $foo.baz; # «Any␤»
+
 =head1 Methods
 
 The usual way to obtain an object of type C<Attribute> is by introspection: