Improved GridLayout docs. (#6368)

Co-authored-by: Simon Hausmann <simon.hausmann@slint.dev>

docs/reference/src/language/builtins/elements.md

@@ -253,18 +253,59 @@ export component Example inherits Window {
 
 ## `GridLayout`
 
-`GridLayout` places its children in a grid. `GridLayout` adds properties to each child: `col`, `row`, `colspan`, `rowspan`.
+`GridLayout` places elements on a grid.
-These properties must be compile time constants.
+
-You can control the position of children with `col` and `row`.
+Cell elements inside a `GridLayout` obtain the following new properties. Any bindings to these properties must be compile-time constants:
-If `col` or `row` aren't specified, they are automatically computed such that the item is next to the previous item, in the same row.
+
-Alternatively, the item can be put in a `Row` element.
+-   **`row`** (_in_ _int_): The index of the element's row within the grid. Setting this property resets the element's column to zero, unless explicitly set.
+-   **`col`** (_in_ _int_): The index of the element's column within the grid. Set this property to override the sequential column assignment (e.g., to skip a column).
+-   **`rowspan`** (_in_ _int_): The number of rows this element should span. (default value: `1`)
+-   **`colspan`** (_in_ _int_): The number of columns this element should span. (default value: `1`)
+
+To implicitly sequentially assign row indices—just like with `col`—wrap cell elements in `Row` elements.
+
+The following example creates a 2-by-2 grid with `Row` elements, omitting one cell:
+
+```slint
+import { Button } from "std-widgets.slint";
+export component Foo inherits Window {
+    width: 200px;
+    height: 100px;
+    GridLayout {
+        Row { // children implicitly on row 0
+            Button { col: 1; text: "Top Right"; } // implicit column after this would be 2
+        }
+        Row { // children implicitly on row 1
+            Button { text: "Bottom Left"; }  // implicitly in column 0...
+            Button { text: "Bottom Right"; } // ...and 1
+        }
+    }
+}
+```
+
+The following example creates the same grid using the `row` property. Row indices must be taken care of manually:
+
+```slint
+import { Button } from "std-widgets.slint";
+export component Foo inherits Window {
+    width: 200px;
+    height: 100px;
+    GridLayout {
+        Button { row: 0; col: 1; text: "Top Right"; } // `row: 0;` could even be left out at the start
+        Button { row: 1; text: "Bottom Left"; } // new row, implicitly resets column to 0
+        Button { text: "Bottom Right"; } // same row, sequentially assigned column 1
+    }
+}
+```
+
+`GridLayout` covers its entire surface with cells. Cells are not aligned. The elements constituting the cells will be stretched inside their allocated space, unless their size constraints—like, e.g., `min-height` or `max-width`—work against this.
 
 ### Properties
 
 -   **`spacing`** (_in_ _length_): The distance between the elements in the layout.
 -   **`spacing-horizontal`**, **`spacing-vertical`** (_in_ _length_):
-    Set these properties to override the spacing on specific directions.
+    Set these properties to override the spacing on specific axes.
--   **`padding`** (_in_ _length_): The padding within the layout.
+-   **`padding`** (_in_ _length_): The padding around the grid structure as a whole.
 -   **`padding-left`**, **`padding-right`**, **`padding-top`** and **`padding-bottom`** (_in_ _length_):
     Set these properties to override the padding on specific sides.