Split sub defined and method defined.

Fix issue #1800.

Author: fluca1978

doc/Type/Mu.pod6

@@ -18,7 +18,16 @@ L<Any|/type/Any>.
 
 =head2 routine defined
 
+Defined as
+
     multi sub    defined(Mu --> Bool:D)
+
+invokes the C<.defined> method on the object and returns its result.
+
+=head2 method defined
+
+Defined as
+
     multi method defined(   --> Bool:D)
 
 Returns C<False> on the type object, and C<True> otherwise.