Update SliverPersistentHeader docs (#46081)

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

@@ -91,8 +91,11 @@ abstract class SliverPersistentHeaderDelegate {
   bool shouldRebuild(covariant SliverPersistentHeaderDelegate oldDelegate);
 }
 
-/// A sliver whose size varies when the sliver is scrolled to the leading edge
-/// of the viewport.
+/// A sliver whose size varies when the sliver is scrolled to the edge
+/// of the viewport opposite the sliver's [GrowthDirection].
+///
+/// In the normal case of a [CustomScrollView] with no centered sliver, this
+/// sliver will vary its size when scrolled to the leading edge of the viewport.
 ///
 /// This is the layout primitive that [SliverAppBar] uses for its
 /// shrinking/growing effect.