keyboard: Fix comment regarding implicit calls to SDL_StartTextInput()

It was always done inside SDL_VideoInit().

(cherry picked from commit 92cadaf917)

include/SDL_keyboard.h

@@ -255,7 +255,7 @@ extern DECLSPEC SDL_Keycode SDLCALL SDL_GetKeyFromName(const char *name);
  * On some platforms using this function activates the screen keyboard.
  *
  * On desktop platforms, SDL_StartTextInput() is implicitly called on SDL
- * window creation which will cause events SDL_TextInputEvent and
+ * video subsystem initialization which will cause SDL_TextInputEvent and
  * SDL_TextEditingEvent to begin emitting.
  *
  * \since This function is available since SDL 2.0.0.