Add a comment about not obvious code in VariantData.

Change-Id: I943af28c47b396aa35173da2a1294b86c8a522fa
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qmetatype.h

@@ -777,6 +777,7 @@ struct VariantData
     const void *data;
     const uint flags;
 private:
+    // copy constructor allowed to be implicit to silence level 4 warning from MSVC
     VariantData &operator=(const VariantData &) Q_DECL_EQ_DELETE;
 };