Polarity issue in JFET transistor
Seems to me that you have seen a translation error. It's not that JFETs are 'prohibited' to be polarized, it's just reverse biased. Well, if you bias it directly it doesn't work…
The JFET is a depletion device and in fact it 'pinch off' (block the flow of the current) when the gate is at  some voltage below the source (for the N-channel, how much is part dependand). Your second definition has the absolute value signs because for P-channel most signs are reversed.
By the way, be careful: the saturation zone in a FET (junction or MOS) is completely different than the saturation of a BJT (in fact, it's its linear zone…). It's a bad naming that confuses a lot of people.
Up (or Vp) is the pinch off voltage, the threshold of total depletion. If you reverse bias the gate over that, no current passes (there's always some leakage and parasitic stuff, but that's the idea). When the channel is open, the Uds (Vds) determines the kind of response it has on the signal (mostly if amplifies or works like a variable resistor, read the rest of your page)