I'm not entirely sure I understand your question.
The currently language formally requires the specified points to be three of the four corners of a rectangle. It doesn't actually state this, and I don't have any real objection to calling that out, if that's what you're asking for. However, since P1 and P2 are are both perpendicular to each other and to the texture plane, the input points must by definition be three out of four corners of a rectangle (unless I'm mis-reading).
The currently language formally requires the specified points to be three of the four corners of a rectangle. It doesn't actually state this, and I don't have any real objection to calling that out, if that's what you're asking for. However, since P1 and P2 are are both perpendicular to each other and to the texture plane, the input points must by definition be three out of four corners of a rectangle (unless I'm mis-reading).