Add to TableCell docs (#138258)

Fixes https://github.com/flutter/flutter/issues/42523
Came across a trivial doc issue - now it will be fixed. 😁

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

@@ -397,6 +397,9 @@ class _TableElement extends RenderObjectElement {
 /// the [TableCell] widget to its enclosing [Table] must contain only
 /// [TableRow]s, [StatelessWidget]s, or [StatefulWidget]s (not
 /// other kinds of widgets, like [RenderObjectWidget]s).
+///
+/// To create an empty [TableCell], provide a [SizedBox.shrink]
+/// as the [child].
 class TableCell extends ParentDataWidget<TableCellParentData> {
   /// Creates a widget that controls how a child of a [Table] is aligned.
   const TableCell({