MultiSelectableSelectionContainerDelegate documentation fixes. (#147843)

Minor grammatical fixes to MultiSelectableSelectionContainerDelegate:

* Removed plural words describing a singular subject.
* Removed redundant language. 
* Elaborated on the behavior of `ensureChildUpdated`.

No issue was filed this was a minor rewording.

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

@@ -1761,14 +1761,12 @@ class _SelectableRegionContainerDelegate extends MultiSelectableSelectionContain
   }
 }
 
-/// An abstract base class for updating multiple [Selectable] children.
+/// A delegate that handles events and updates for multiple [Selectable]
+/// children.
 ///
-/// This class provide basic [SelectionEvent] handling and child [Selectable]
-/// updating. The subclass needs to implement [ensureChildUpdated] to ensure
-/// child [Selectable] is updated properly.
-///
-/// This class optimize the selection update by keeping track of the
-/// [Selectable]s that currently contain the selection edges.
+/// Updates are optimized by tracking which [Selectable]s reside on the edges of
+/// a selection. Subclasses should implement [ensureChildUpdated] to describe
+/// how a [Selectable] should behave when added to a selection.
 abstract class MultiSelectableSelectionContainerDelegate extends SelectionContainerDelegate with ChangeNotifier {
   /// Creates an instance of [MultiSelectableSelectionContainerDelegate].
   MultiSelectableSelectionContainerDelegate() {