efl_ui/flox_box: improve docs

Summary:
expand the description for the class a bit

ref T7717

Reviewers: segfaultxavi

Reviewed By: segfaultxavi

Subscribers: cedric, #reviewers, #committers

Tags: #efl_docs

Maniphest Tasks: T7717

Differential Revision: https://phab.enlightenment.org/D9885

src/lib/elementary/efl_ui_box_flow.eo

@@ -1,6 +1,11 @@
 class @beta Efl.Ui.Box_Flow extends Efl.Ui.Box
 {
-   [[A custom layout engine for @Efl.Ui.Box.]]
+   [[A Flow Box is a customized type of @Efl.Ui.Box. It will fill along the given axis
+     until items will no longer fit in the available space, at which point it will begin
+     filling items in a new row/column after the current one. This is useful if an application
+     wants to e.g., present a group of items and wrap them onto subsequent lines when the number
+     of items grows too large to fit on the screen.
+   ]]
    implements {
       Efl.Pack_Layout.layout_update;
    }