Fix SliverList example descriptions (flutter#134483)

Fixes flutter#134142

The description for the SliverList.list example was just wrong, describing something other than the sample. � 
Tweaked some of the language on the others while I was here.

Author: Piinks

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

@@ -177,8 +177,8 @@ class SliverList extends SliverMultiBoxAdaptorWidget {
   /// [SliverChildBuilderDelegate.addSemanticIndexes] property.
   ///
   /// {@tool snippet}
-  /// This example, which would be inserted into a [CustomScrollView.slivers]
-  /// list, shows an infinite number of items in varying shades of blue:
+  /// This example, which would be provided in [CustomScrollView.slivers],
+  /// shows an infinite number of items in varying shades of blue:
   ///
   /// ```dart
   /// SliverList.builder(
@@ -236,10 +236,11 @@ class SliverList extends SliverMultiBoxAdaptorWidget {
   /// [SliverChildBuilderDelegate.addRepaintBoundaries] property. The
   /// `addSemanticIndexes` argument corresponds to the
   /// [SliverChildBuilderDelegate.addSemanticIndexes] property.
-  /// {@tool snippet}
   ///
+  /// {@tool snippet}
   /// This example shows how to create a [SliverList] whose [Container] items
-  /// are separated by [Divider]s.
+  /// are separated by [Divider]s. The [SliverList] would be provided in
+  /// [CustomScrollView.slivers].
   ///
   /// ```dart
   /// SliverList.separated(
@@ -303,8 +304,8 @@ class SliverList extends SliverMultiBoxAdaptorWidget {
   /// [SliverChildBuilderDelegate.addSemanticIndexes] property.
   ///
   /// {@tool snippet}
-  /// This example, which would be inserted into a [CustomScrollView.slivers]
-  /// list, shows an infinite number of items in varying shades of blue:
+  /// This example, which would be provided in [CustomScrollView.slivers],
+  /// shows a list containing two [Text] widgets:
   ///
   /// ```dart
   /// SliverList.list(