[6.d] Document DefiniteHOW's new default defaults

Per 6.d-prep: https://github.com/perl6/6.d-prep/blob/dffa2642419a0f481591b9b64b602bfc4c4eb66b/TODO/FEATURES.md#make-default-defaults-for-definitehows-be-normal-types
POV: rakudo/rakudo@38b198c992
Propspec: Raku/roast@8c8e8f55bb

doc/Type/Signature.pod6

@@ -373,6 +373,20 @@ which is in part clarified by the different way default values are
 declared in each case (using C<=> for parameters, using the C<default>
 trait for variables).
 
+Note: in 6.c language, the default default of C<:U>/C<:D> constrained
+variables was a type object with such a constraint, which is not initializeable,
+thus you cannot use the C<.=> operator, for example.
+
+    use v6.c;
+    my Int:D $x .= new: 42;
+    # OUTPUT: You cannot create an instance of this type (Int:D)
+    # in block <unit> at -e line 1
+
+In 6.d language, the default default is the type object without the smiley constraint:
+
+    use v6.d.PREVIEW;
+    my Int:D $x .= new: 42; # OUTPUT: «42␤»
+
 =head3 Constraining signatures of C<Callable>s
 
 The signature of a L<Callable> parameter can be constrained by