Add docs for RenderSliverGrid (#8737)

Most of these docs were already in place, but this patch adds the remaining
missing ones.

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

@@ -227,6 +227,12 @@ abstract class SliverGridDelegate {
   /// Returns information about the size and position of the tiles in the grid.
   SliverGridLayout getLayout(SliverConstraints constraints);
 
+  /// Override this method to return true when the children need to be
+  /// laid out.
+  ///
+  /// This should compare the fields of the current delegate and the given
+  /// `oldDelegate` and return true if the fields are such that the layout would
+  /// be different.
   bool shouldRelayout(covariant SliverGridDelegate oldDelegate);
 }
 
@@ -401,20 +407,44 @@ class SliverGridDelegateWithMaxCrossAxisExtent extends SliverGridDelegate {
   }
 }
 
+/// Parent data structure used by [RenderSliverGrid].
 class SliverGridParentData extends SliverMultiBoxAdaptorParentData {
+  /// The offset of the child in the non-scrolling axis.
+  ///
+  /// If the scroll axis is vertical, this offset is from the left-most edge of
+  /// the parent to the left-most edge of the child. If the scroll axis is
+  /// horizontal, this offset is from the top-most edge of the parent to the
+  /// top-most edge of the child.
   double crossAxisOffset;
 
   @override
   String toString() => 'crossAxisOffset=$crossAxisOffset; ${super.toString()}';
 }
 
+/// A sliver that contains multiple box children that whose size and position
+/// are determined by a delegate.
+///
+/// [RenderSliverGrid] places its children in arbitrary positions determined by
+/// [gridDelegate]. Each child is forced to have the size specified by the
+/// [gridDelegate].
+///
+/// See also:
+///
+///  * [RenderSliverList], which places its children in a linear
+///    array.
+///  * [RenderSliverFixedExtentList], which places its children in a linear
+///    array with a fixed extent in the main axis.
 class RenderSliverGrid extends RenderSliverMultiBoxAdaptor {
+  /// Creates a sliver that contains multiple box children that whose size and
+  /// position are determined by a delegate.
+  ///
+  /// The [childManager] and [gridDelegate] arguments must not be null.
   RenderSliverGrid({
     @required RenderSliverBoxChildManager childManager,
     @required SliverGridDelegate gridDelegate,
   }) : _gridDelegate = gridDelegate,
        super(childManager: childManager) {
-    gridDelegate != null;
+    assert(gridDelegate != null);
   }
 
   @override
@@ -423,9 +453,9 @@ class RenderSliverGrid extends RenderSliverMultiBoxAdaptor {
       child.parentData = new SliverGridParentData();
   }
 
+  /// The delegate that controls the size and position of the children.
   SliverGridDelegate get gridDelegate => _gridDelegate;
   SliverGridDelegate _gridDelegate;
-
   set gridDelegate(SliverGridDelegate value) {
     assert(value != null);
     if (_gridDelegate == value)