I know what a Reset signal is, but a Set/Reset signal? Isn't that impossible?
In all likelihood it isn't really a "set/reset" signal, so much as a signal that can be configured to be either set or reset depending on how the device is configured. Note that in figure 3.2 (copied below for clarity), it says "flip-flop with optional [...] set or reset controls".

Similarly it will also be the configuration bits that disable the signal entirely if unused. You can see this in figure 3.2: