TDD is about documenting behaviour. Which is why it was later given the name Behaviour Driven Development (BDD), to dispel the myths that it is about testing. It is true that you need to document behaviour before writing code, else how would you know what to write? Even outside of TDD you need to document the behaviour some way before you can know what needs to be written.

A function's behaviour should have no reason to change after its behaviour is documented. You can change the implementation beneath to your hearts content, but the behaviour should be static. If someone attempts to change the behaviour, you are going to know about it. If you are not alerted to this, your infrastructure needs improvement.