Clarify the whole "CustomPainters default to Size.zero" thing. (#130624)

Fixes https://github.com/flutter/flutter/issues/52707

packages/flutter/lib/src/widgets/basic.dart

@@ -600,11 +600,13 @@ class BackdropFilter extends SingleChildRenderObjectWidget {
 /// `setState` or `markNeedsLayout` during the callback (the layout for this
 /// frame has already happened).
 ///
-/// Custom painters normally size themselves to their child. If they do not have
-/// a child, they attempt to size themselves to the [size], which defaults to
-/// [Size.zero]. [size] must not be null.
+/// Custom painters normally size themselves to their [child]. If they do not
+/// have a child, they attempt to size themselves to the specified [size], which
+/// defaults to [Size.zero]. The parent [may enforce constraints on this
+/// size](https://docs.flutter.dev/ui/layout/constraints).
 ///
-/// [isComplex] and [willChange] are hints to the compositor's raster cache.
+/// The [isComplex] and [willChange] properties are hints to the compositor's
+/// raster cache.
 ///
 /// {@tool snippet}
 ///