Update SliverChildDelegate docs (#97674)

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

@@ -106,6 +106,14 @@ int _kDefaultSemanticIndexCallback(Widget _, int localIndex) => localIndex;
 ///    using the [AutomaticKeepAliveClientMixin], then implementing the
 ///    [AutomaticKeepAliveClientMixin.wantKeepAlive] getter and calling
 ///    [AutomaticKeepAliveClientMixin.updateKeepAlive].
+///
+/// ## Using more than one delegate in a [Viewport]
+///
+/// If multiple delegates are used in a single scroll view, the first child of
+/// each delegate will always be laid out, even if it extends beyond the
+/// currently viewable area. This is because at least one child is required in
+/// order to [estimateMaxScrollOffset] for the whole scroll view, as it uses the
+/// currently built children to estimate the remaining children's extent.
 /// {@endtemplate}
 ///
 /// See also:
@@ -788,6 +796,15 @@ abstract class SliverMultiBoxAdaptorWidget extends SliverWithKeepAliveWidget {
   /// The children are constructed lazily using this delegate to avoid creating
   /// more children than are visible through the [Viewport].
   ///
+  /// ## Using more than one delegate in a [Viewport]
+  ///
+  /// If multiple delegates are used in a single scroll view, the first child of
+  /// each delegate will always be laid out, even if it extends beyond the
+  /// currently viewable area. This is because at least one child is required in
+  /// order to estimate the max scroll offset for the whole scroll view, as it
+  /// uses the currently built children to estimate the remaining children's
+  /// extent.
+  ///
   /// See also:
   ///
   ///  * [SliverChildBuilderDelegate] and [SliverChildListDelegate], which are