Document that you may receive mouse events with SDL_PEN_MOUSEID

(cherry picked from commit 716c767b7e7099fb0e2fda9db59227a7e1d1d8a4)

include/SDL3/SDL_events.h

@@ -456,7 +456,7 @@ typedef struct SDL_MouseMotionEvent
     Uint32 reserved;
     Uint64 timestamp;   /**< In nanoseconds, populated using SDL_GetTicksNS() */
     SDL_WindowID windowID; /**< The window with mouse focus, if any */
-    SDL_MouseID which;  /**< The mouse instance id in relative mode, SDL_TOUCH_MOUSEID for touch events, or 0 */
+    SDL_MouseID which;  /**< The mouse instance id in relative mode, SDL_TOUCH_MOUSEID for touch events, SDL_PEN_MOUSEID for pen events, or 0 */
     SDL_MouseButtonFlags state;       /**< The current button state */
     float x;            /**< X coordinate, relative to window */
     float y;            /**< Y coordinate, relative to window */