Clarify comment. Closes #1631.

Source/Urho3D/Core/Variant.h

@@ -63,7 +63,7 @@ enum VariantType
     MAX_VAR_TYPES
 };
 
-/// Union for the possible variant values. Also stores non-POD objects such as String and math objects (excluding Matrix) which must not exceed 16 bytes in size. Objects exceeding 16 bytes size are stored in the heap pointed by _ptr.
+/// Union for the possible variant values. Also stores non-POD objects such as String and math objects (excluding Matrix) which must not exceed 16 bytes in size (or 32 bytes in a 64-bit build.) Objects exceeding the limit are allocated on the heap and pointed to by _ptr.
 struct VariantValue
 {
     union