It affects both rising and falling edges.
 
Devices that support Slew Rate
・ Arria II GX, Cyclone III, Cyclone IV, HardCopy III, HardCopy IV, Stratix III, Stratix IV
 
Devices that support Slow Slew Rate
・ Cyclone, MAX II, MAX 3000, MAX 7000, Stratix, Stratix GX

 
The Slew Rate options are
0 : slow slew rate
3: fast slew rate
*Cyclone III and Cyclone IV up to fast slew rate 2
 
The Slow Slew Rate options are
ON : slow slew rate
OFF: Fast slew rate
 
For detailed functions, search for the keyword "Slew Rate" in the Device Handbook of each device.