docs(indev): add comment in input device part
To register an input device an `lv_indev_drv_t` variable has to be initialized. **Be sure to register at least one display before you register any input devices.**
```c
+/*Register at least one display before you register any input devices*/
lv_disp_drv_register(&disp_drv);
static lv_indev_drv_t indev_drv;
