Add missing AbsoluteLayout section to layout architecture doc (#19841)

docs/design/layout.md

@@ -269,9 +269,43 @@ As an example, take a Grid with `HeightRequest="100"` and `RowDefinitions="*,2*,
 
 ## AbsoluteLayout
 
-TK
+AbsoluteLayout allows child views to be positioned and sized using explicit values and/or proportionally to the size of the layout.
 
+The layout of a child view in AbsoluteLayout can be specified with a combination of two values:
 
+### LayoutBounds
+
+The layout bounds of a child view is a `Rect` which specifies the position and size of the view. If not specified, the default layout bounds for a view are a position of (0, 0) with the `Width` and `Height` properties set to `AbsoluteLayout.AutoSize` (a constant value of -1). 
+
+### LayoutFlags
+
+The layout flags of a child view specify how the values in the layout bounds are used. By default, no flags are specified (`AbsoluteLayoutFlags.None`). The `XProportional` and `YProportional` values can be independently set, or both values can be set at once by setting `PositionProportional`. Likewise, `HeightProportional` and `WidthProportional` can be set independently, or both can be set at once with `SizeProportional`. All flags can be set at once using `AbsoluteLayoutFlags.All`.
+
+### Layout Bounds/Flags Interaction
+
+The layout rules for a child view are determined by a combination of its layout bounds and its layout flags:
+
+#### Position
+
+If the `XProportional`/`YProportional` flags are set, then the X/Y values of the layout bounds are multiplied by the width/height of the AbsoluteLayout to determine the final X/Y coordinate values. For instance, imagine an AbsoluteLayout which is 100 x 100. It has a child with layout bounds of (0.4, 0.6, 20, 20), and the child's `XProportional` and `YProportional` flags are set. The position of that child will be X = (100 * 0.4) = 40 and Y = (100 * 0.6) = 60. 
+
+If `XProportional`/`YProportional` flags are not set, then the X and Y values in the layout bounds are explicit values. For instance, imagine an AbsoluteLayout which is 100 x 100. It has a child with layout bounds of (45, 67, 20, 20), and the child's `XProportional` and `YProportional` flags are  _not_ set. The position of that child will be X = 45 and Y = 67. 
+
+Both explicit and proportional position values _can_ exceed the boundaries of the AbsoluteLayout.
+
+#### Size
+
+If the `WidthProportional`/`HeightProportional` flags are set, then the Width/Height values of the layout bounds are multiplied by the width/height of the AbsoluteLayout to determine the final Width/Height values. For instance, imagine an AbsoluteLayout which is 100 x 100. It has a child with layout bounds of (0, 0, 0.3, 0.47), and the child's `WidthProportional` and `HeightProportional` flags are set. The size of that child will be Width = (100 * 0.3) = 30 and Height = (100 * 0.47) = 47. 
+
+If `WidthProportional`/`HeightProportional` flags are not set, then the Width and Height values in the layout bounds are explicit values. For instance, imagine an AbsoluteLayout which is 100 x 100. It has a child with layout bounds of (0, 0, 45, 20), and the child's `WidthProportional` and `HeightProportional` flags are  _not_ set. The size of that child will be Width = 45 and Height = 20. 
+
+Both explicit and proportional size values _can_ exceed the boundaries of the AbsoluteLayout.
+
+### Unbounded Dimensions
+
+If the AbsoluteLayout is unbounded along a dimension (e.g., if it's a child of a VerticalStackLayout, and is not provided with an explicit height), then the concept of "proportional" along that dimension has no meaning. Proportional flags along those dimensions are ignored; if the layout bounds specifies a non-Auto value along that dimension, it will be used. Otherwise, the automatic size of the view will be used along that dimension.
+
+For example, say an AbsoluteLayout is the child of a VerticalStackLayout with a width of 200. The only child of the AbsoluteLayout has LayoutBounds of (0, 0, 100, 0.5) and has `AbsoluteLayoutFlags.HeightProportional` set. Because the AbsoluteLayout has no vertical constraints, the `AbsoluteLayoutFlags.HeightProportional` flag is ignored, and the child's height is set to the value of 0.5 from its layout bounds. 
 
 # The Layout Process