Document infix:<but> with object as RHS

doc/Language/operators.pod6

@@ -1223,14 +1223,22 @@ to act like a role, for example enum values.
 =head2 infix C«but»
 
 =begin code :skip-test
-sub infix:<but>(Mu $obj, Mu $role) is assoc<non>
+sub infix:<but>(Mu $obj, Mu  $role) is assoc<non>
+sub infix:<but>(Mu $obj, Mu:D $obj) is assoc<non>
 =end code
 
 Creates a copy of C<$obj> with C<$role> mixed in. Since C<$obj> is not
 modified, C<but> can be used to created immutable values with mixins.
 
-C<$role> doesn't need to a be a role, it can be something that knows how
-to act like a role, for example enum values.
+Instead of a role, you can provide an instantiated object. In this case,
+the operator will create a role for you automatically. The role will contain
+a single method named the same as C<$obj.^name> and that returns C<$obj>:
+
+    say 42 but 'forty two'; # 'forty two'
+
+    my $s = 12 but class { method ^name ($) { 'warbles' } }.new;
+    say $s.warbles; # warbles.new
+    say $s + 42;    # 54
 
 =head2 infix C«cmp»