Update Data classes.md

[ossner]

I wanted to clarify some things on this page which might help people understand more of the concepts:

• Show that the data class can override any default method (such as equals) as well
• Make it clear that instances' hashCodes are only equal iff they have exactly the same attributes, not if the objects are structurally equal
• Show that the copy method really does create a new instance by checking referential equality with the copied object (- Also remove () from one of the hashCodes in the plain text to make it consistent)

[p7nov]

Thank you for this improvement!