Add a note about sizedByParent's use. One day we should audit our RenderBoxes and see if any of the…

sky/sdk/lib/rendering/README.md

 Sky Rendering
 =============
 
 The Sky render tree is a low-level layout and painting system based on a
 retained tree of objects that inherit from [`RenderObject`](object.dart). Most
 developers using Sky will not need to interact directly with the rendering tree.
 Instead, most developers should use [Sky widgets](../widgets/README.md), which
 are built using the render tree.
 
 Overview
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    `size` member. The size must be consistent, a given set of
    constraints must always result in the same size.
 
    In the latter case, it will inherit the default `sizedByParent`
    getter that returns false, and it will size itself in the
    `performLayout()` function described below.
 
    The `sizedByParent` distinction is purely a performance
    optimisation. It allows nodes that only set their size based on the
    incoming constraints to skip that logic when they need to be
-   re-laid-out.
+   re-laid-out, and, more importantly, it allows the layout system to
+   treat the node as a _layout boundary_, which reduces the amount of
+   work that needs to happen when the node is marked as needing
+   layout.
 
 * The following methods must report numbers consistent with the output
   of the layout algorithm used:
 
 ** `double getMinIntrinsicWidth(BoxConstraints constraints)` must
    return the width that fits within the given constraints below which
    making the width constraint smaller would not increase the
    resulting height, or, to put it another way, the narrowest width at
    which the box can be rendered without failing to lay the children
    out within itself.
@@ skipping 86 matching lines @@
 
 * Avoid using save/restore on canvases.
 
 
 Dependencies
 ------------
 
  * [`package:sky/base`](../base)
  * [`package:sky/mojo`](../mojo)
  * [`package:sky/animation`](../mojo)