Added documentation for SDL_FLT_EPSILON (thanks @MrOnlineCoder!)

Closes https://github.com/libsdl-org/sdlwiki/pull/549

include/SDL3/SDL_stdinc.h

@@ -279,6 +279,9 @@ typedef Sint64 SDL_Time;
  */
 /* @{ */
 
+/**
+ * Epsilon constant, used for comparing floating-point numbers. Equals by default to platform-defined FLT_EPSILON, or 1.1920928955078125e-07F if that's not available.
+ */
 #ifdef FLT_EPSILON
 #define SDL_FLT_EPSILON FLT_EPSILON
 #else