Added atan2 function to api-c-stdlib docs

Author: JohnDog3112

docs/api/api-c-stdlib.md

@@ -108,6 +108,7 @@ void assert(int expression);
 double acos(double arg);
 double asin(double arg);
 double atan(double arg);
+double atan2(double y, double x);
 double ceil(double arg);
 double cos(double arg);
 double exp(double arg);