v4 doc: fixed reference to non-existent function

(Fixed to remove accidental commit of another change)

After some feedback from Marek Chalupa I decided to just remove this. There
were suggestions about warning about multiple threads but it appears this
would be true for many of these functions and thus it would be misleading to
mention multiple threads only here (as it would imply that multiple threads
work for other functions which is not true, I think).

Acked-by: Marek Chalupa <mchqwerty@gmail.com>