a place to start: n channel j fet:
http://www.learningaboutelectronics.com/Articles/N-channel-JFET
the diodes are an opportunity to protect the gate
for example, consider the SOT353 case:
pin 2 is the gate.
by connecting pin 1 to "something", the gate is "clamped" to no more than
a diode drop from the "something"
if you connect pin 1 to the source, pin 5, the gate cannot be more than 1 diode
drop above or below the source voltage
note that on page 2 of the spec sheet, the gate diode voltage is 0.4 to 0.8 V
so if pin 1 is connected to "something" the gate will not go more that 0.4 to 0.8 V above "something"
nor will it go more that 0.4 to 0.8 V below "something"
i don't see any use for the diodes.
perhaps someone wiser than i has a better answer
i suggest you contact InterFET and ask