Fixed documentation for SDL_JoystickGetDeviceInstanceID

Fixes https://github.com/libsdl-org/SDL/issues/6946

include/SDL_joystick.h

@@ -288,13 +288,12 @@ extern DECLSPEC SDL_JoystickType SDLCALL SDL_JoystickGetDeviceType(int device_in
 /**
  * Get the instance ID of a joystick.
  *
- * This can be called before any joysticks are opened. If the index is out of
- * range, this function will return -1.
+ * This can be called before any joysticks are opened.
  *
  * \param device_index the index of the joystick to query (the N'th joystick
  *                     on the system
  * \returns the instance id of the selected joystick. If called on an invalid
- *          index, this function returns zero
+ *          index, this function returns -1.
  *
  * \since This function is available since SDL 2.0.6.
  */