Tell people to not use their PIN as name?

I recently noticed that in the WhatsApp profile section, below the text field for changing my name, there is a hint saying that this is neither my username nor my pin. The form looks like the following mockup (the label "Your name" is only visible in the web version):

^{download bmml source – Wireframes created with Balsamiq Mockups}

My concern is that this hint will cause more people to put their PIN into the field as people don't always read all information on a page that carefully and without the hint their PIN probably wouldn't even come to their mind. On the other hand one could argue that people think twice about putting their confidential information into a form, but then I still don't see any reason for the hint.

Has anyone done related research on this or are there even better ways of preventing users from putting confidential information to their publicly available profile information?