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docs(scroll) add information about scroll coordinates (#3088)

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docs/overview/scroll.md
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@ -141,6 +141,29 @@ The following API functions allow manual scrolling of objects:
- `lv_obj_scroll_to_x(obj, x, LV_ANIM_ON/OFF)` scroll to bring the given coordinate to the left side
- `lv_obj_scroll_to_y(obj, y, LV_ANIM_ON/OFF)` scroll to bring the given coordinate to the top side
From time to time you may need to retrieve the scroll position of an element, either to restore it later, or to display dynamically some elements according to the current scroll.
Here is an example to see how to combine scroll event and store the scroll top position.
```c
static int scroll_value = 0;
static void store_scroll_value_event_cb(lv_event_t* e) {
lv_obj_t* screen = lv_event_get_target(e);
scroll_value = lv_obj_get_scroll_top(screen);
printf("%d pixels are scrolled out on the top\n", scroll_value);
}
lv_obj_t* container = lv_obj_create(NULL);
lv_obj_add_event_cb(container, store_scroll_value_event_cb, LV_EVENT_SCROLL, NULL);
```
Scrool coordinates can be retrieve from differents axes with these functions:
- `lv_obj_get_scroll_x(obj)` Get the `x` coordinate of object
- `lv_obj_get_scroll_y(obj)` Get the `y` coordinate of object
- `lv_obj_get_scroll_top(obj)` Get the scroll coordinate from the top
- `lv_obj_get_scroll_bottom(obj)` Get the scroll coordinate from the bottom
- `lv_obj_get_scroll_left(obj)` Get the scroll coordinate from the left
- `lv_obj_get_scroll_right(obj)` Get the scroll coordinate from the right
## Self size
Self size is a property of an object. Normally, the user shouldn't use this parameter but if a custom widget is created it might be useful.