docs: Add documentation for Efl.Ui.Scroller

Summary:
These docs were missing.
Ref T7891

Test Plan: Only docs changed

Reviewers: zmike, woohyun, bu5hm4n

Reviewed By: zmike

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Maniphest Tasks: T7891

Differential Revision: https://phab.enlightenment.org/D10019

src/lib/elementary/efl_ui_scroller.eo

@@ -6,7 +6,18 @@ class @beta Efl.Ui.Scroller extends Efl.Ui.Layout_Base implements
                        Efl.Ui.Scrollable,
                        Efl.Ui.Scrollbar
 {
-   [[Efl ui scroller class]]
+   [[Widget container that allows objects bigger than itself to be put inside it, and provides
+     scrolling functionality so the whole content is visible.
+   
+     Some widgets have scrolling capabilities (like @Efl.Ui.List) that allow big content
+     to be shown inside a small viewport, using the well-known scrollbar objects.
+     Some other widgets (like @Efl.Ui.Box, for example) cannot scroll by themselves and
+     therefore would not be fully visible when placed inside a viewport smaller than them.
+
+     The @Efl.Ui.Scroller is a helper class that provides scrolling capabilities for widgets
+     which don't have them. In the above example, simply putting the @Efl.Ui.Box inside a
+     @Efl.Ui.Scroller (using @Efl.Content.content.set) would give it the ability to scroll.
+   ]]
    implements {
       Efl.Object.constructor;
       Efl.Object.finalize;