What does 'undocumented' mean
It means that it's not documented anywhere
because it's an implementation detail that is not part of the public API, and on which users should not rely.
why would that be changed?
It's an implementation detail, therefore I can touch it without breaking the API compatibility between minor versions of SFML.
Because a member (function)
can be accessed doesn't mean that it
should be used. Users should strictly stick to what's publicly documented.