I just now found out about the existence of sprintf() in Arduino. Why is it not documented?
I went to google and type sprintf. I got
About 4,710,000 results (0.10 seconds)
Looks to me like it's documented.
If you mean why isn't it described here:http://arduino.cc/en/Reference/HomePage
It's because the Arduino is programmed in C++ which is a superset of C, so (almost) every C function works on Arduino. There are exceptions, like fopen(), where the function is dependent on resources that the Arduino doesn't have, but there aren't that many of them.