Add more dartdocs for slivers (#9164)

This patch adds docs for many of the sliver widgets.

packages/flutter/lib/src/rendering/sliver_fixed_extent_list.dart

@@ -211,8 +211,8 @@ abstract class RenderSliverFixedExtentBoxAdaptor extends RenderSliverMultiBoxAda
   }
 }
 
-/// A sliver that contains multiple box children that have a given extent in the
-/// main axis.
+/// A sliver that places multiple box children with the same main axis extent in
+/// a linear array.
 ///
 /// [RenderSliverFixedExtentList] places its children in a linear array along
 /// the main axis starting at offset zero and without gaps. Each child is forced

packages/flutter/lib/src/rendering/sliver_grid.dart

@@ -424,8 +424,7 @@ class SliverGridParentData extends SliverMultiBoxAdaptorParentData {
   String toString() => 'crossAxisOffset=$crossAxisOffset; ${super.toString()}';
 }
 
-/// A sliver that contains multiple box children that whose size and position
-/// are determined by a delegate.
+/// A sliver that places multiple box children in a two dimensional arrangement.
 ///
 /// [RenderSliverGrid] places its children in arbitrary positions determined by
 /// [gridDelegate]. Each child is forced to have the size specified by the

packages/flutter/lib/src/rendering/sliver_list.dart

@@ -24,8 +24,9 @@ import 'sliver_multi_box_adaptor.dart';
 ///
 /// If the children have a fixed extent in the main axis, consider using
 /// [RenderSliverFixedExtentList] rather than [RenderSliverList] because
-/// does not need to perform layout on its children to obtain their extent in
-/// the main axis and is therefore more efficient.
+/// [RenderSliverFixedExtentList] does not need to perform layout on its
+/// children to obtain their extent in the main axis and is therefore more
+/// efficient.
 ///
 /// See also:
 ///

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

@@ -68,8 +68,7 @@ abstract class ScrollView extends StatelessWidget {
   /// Defaults to [Axis.vertical].
   final Axis scrollDirection;
 
-  /// Whether the scroll view scrolls in the reading direction in the
-  /// [scrollDirection].
+  /// Whether the scroll view scrolls in the reading direction.
   ///
   /// For example, if the reading direction is left-to-right and
   /// [scrollDirection] is [Axis.horizontal], then the scroll view scrolls from

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

@@ -38,6 +38,7 @@ import 'scrollable.dart';
 /// * [PageView], for a scrollable that works page by page.
 /// * [Scrollable], which handles arbitrary scrolling effects.
 class SingleChildScrollView extends StatelessWidget {
+  /// Creates a box in which a single widget can be scrolled.
   SingleChildScrollView({
     Key key,
     this.scrollDirection: Axis.vertical,
@@ -57,12 +58,32 @@ class SingleChildScrollView extends StatelessWidget {
     );
   }
 
+  /// The axis along which the scroll view scrolls.
+  ///
+  /// Defaults to [Axis.vertical].
   final Axis scrollDirection;
 
+  /// Whether the scroll view scrolls in the reading direction.
+  ///
+  /// For example, if the reading direction is left-to-right and
+  /// [scrollDirection] is [Axis.horizontal], then the scroll view scrolls from
+  /// left to right when [reverse] is false and from right to left when
+  /// [reverse] is true.
+  ///
+  /// Similarly, if [scrollDirection] is [Axis.vertical], then scroll view
+  /// scrolls from top to bottom when [reverse] is false and from bottom to top
+  /// when [reverse] is true.
+  ///
+  /// Defaults to false.
   final bool reverse;
 
+  /// The amount of space by which to inset the child.
   final EdgeInsets padding;
 
+  /// An object that can be used to control the position to which this scroll
+  /// view is scrolled.
+  ///
+  /// Must be null if [primary] is true.
   final ScrollController controller;
 
   /// Whether this is the primary scroll view associated with the parent
@@ -75,8 +96,15 @@ class SingleChildScrollView extends StatelessWidget {
   /// not specified.
   final bool primary;
 
+  /// How the scroll view should respond to user input.
+  ///
+  /// For example, determines how the scroll view continues to animate after the
+  /// user stops dragging the scroll view.
+  ///
+  /// Defaults to matching platform conventions.
   final ScrollPhysics physics;
 
+  /// The widget that scrolls.
   final Widget child;
 
   AxisDirection _getDirection(BuildContext context) {