Where in the C++11 standard is std::fmodf stated?

According to the cppref page, std::fmodf was added to cmath in C++11. How is this possible though, because wouldn't this mean that cmath would break compatability with math.h previous to C++11? I'm unable to find any references that say std::fmodf was added in C++11 and was wondering where this is stated.

Thank you