Update data-classes.md to avoid making assumption when choosing data class and class

[hieuwu]

🖼 Summary

Currently at the beginning, this page is making assumption that every classes might need some utility functions that comes with data class . However, in some codebase not every class should be marked as data class if the purpose of using these objects does not require any utility functions.
It's good to make a clear understanding when choose using data class and normal class.
In some libraries, they use only class to form a constraint of data to make sure every clients that want to use data from these classes, should only read properties from them and no utility function are required from these classes. In this case, using class is good enough

💡 Update

Add a reminder of usage class or data class should be based on each situation

[sarahhaggarty]

Hi @hieuwu, thanks for your suggestion! After thinking long and hard about this I decided that instead of adding a clarification later like you suggest, it would be better to improve the introductory paragraph. I made this change as part of #3711 Now it should be clear that data classes are a special type of class that come with additional member functions. Thanks for sharing your thoughts with us!