Clarify that the SDL_GetStringRule means that SDL will automatically free the memory later.

include/SDL3/SDL_pen.h

@@ -224,7 +224,7 @@ extern SDL_DECLSPEC SDL_bool SDLCALL SDL_PenConnected(SDL_PenID instance_id);
 /**
  * Retrieves a human-readable description for a SDL_PenID.
  *
- * The returned string follows the SDL_GetStringRule.
+ * The returned string follows the SDL_GetStringRule, and will be automatically freed later.
  *
  * \param instance_id the pen to query.
  * \returns a string that contains the name of the pen, intended for human