Document passing -1 to clear the player index for a controller

include/SDL_gamecontroller.h

@@ -455,7 +455,7 @@ extern DECLSPEC int SDLCALL SDL_GameControllerGetPlayerIndex(SDL_GameController
  * Set the player index of an opened game controller.
  *
  * \param gamecontroller the game controller object to adjust.
- * \param player_index Player index to assign to this controller.
+ * \param player_index Player index to assign to this controller, or -1 to clear the player index and turn off player LEDs.
  *
  * \since This function is available since SDL 2.0.12.
  */