The documentation does call it out explicitly. The message you responded to pointed out where the documentation called it out. You're attacking the authors' intentions and competence when there is no reason for that.
Just saying it needs to be able to fit an unsigned char and call it a day is hardly clear documentation when it isn't even obvious that the signness of the character type is implemention defined for most c-programers.
It almost reads as a deliberate trap (of course it isn't, but imo it is not good documentation).