Minor a11y documentation breadcrumbs (flutter/engine#20330)

Adds a reference to RenderObject.describeSemanticsClip to
`SemanticsFlag.isHidden` and a hint that lets readers know that the
`SemanticsNode` rect is specified in local coordinates.

engine/src/flutter/lib/ui/semantics.dart

@@ -479,6 +479,10 @@ class SemanticsFlag {
   /// the semantics tree altogether. Hidden elements are only included in the
   /// semantics tree to work around platform limitations and they are mainly
   /// used to implement accessibility scrolling on iOS.
+  ///
+  /// See also:
+  ///
+  /// * [RenderObject.describeSemanticsClip]
   static const SemanticsFlag isHidden = SemanticsFlag._(_kIsHiddenIndex);
 
   /// Whether the semantics node represents an image.

engine/src/flutter/lib/ui/semantics/semantics_node.h

@@ -119,8 +119,8 @@ struct SemanticsNode {
   std::string decreasedValue;
   int32_t textDirection = 0;  // 0=unknown, 1=rtl, 2=ltr
 
-  SkRect rect = SkRect::MakeEmpty();
-  SkM44 transform = SkM44{};  // Identity
+  SkRect rect = SkRect::MakeEmpty();  // Local space, relative to parent.
+  SkM44 transform = SkM44{};          // Identity
   std::vector<int32_t> childrenInTraversalOrder;
   std::vector<int32_t> childrenInHitTestOrder;
   std::vector<int32_t> customAccessibilityActions;