Document the range of trigger axes for virtual joysticks

Fixes https://github.com/libsdl-org/SDL/issues/6130

include/SDL_joystick.h

@@ -425,6 +425,8 @@ extern DECLSPEC SDL_bool SDLCALL SDL_JoystickIsVirtual(int device_index);
  * the following: SDL_PollEvent, SDL_PumpEvents, SDL_WaitEventTimeout,
  * SDL_WaitEvent.
  *
+ * Note that when sending trigger axes, you should scale the value to the full range of Sint16. For example, a trigger at rest would have the value of `SDL_JOYSTICK_AXIS_MIN`.
+ *
  * \param joystick the virtual joystick on which to set state.
  * \param axis the specific axis on the virtual joystick to set.
  * \param value the new value for the specified axis.