The Libc (run-time libraries included with the AVR port of GCC) documentation can be found here...
AVR Libc Home Page
Thanks for that! The Language Reference on the Arduino website seems very abbreviated.
sprintf is a function in the C(++) run-time library. It's only connection to C(++) is that it is included with most toolsets.
Actually, sprintf() is part of the official ANSI/ISO C and C++ language standards.