Document tristate value (#110106)

Documentation for Checkbox.value previously did not cover the tristate case.

packages/flutter/lib/src/material/checkbox.dart

@@ -91,7 +91,8 @@ class Checkbox extends StatefulWidget {
 
   /// Whether this checkbox is checked.
   ///
-  /// This property must not be null.
+  /// When [tristate] is true, a value of null corresponds to the mixed state.
+  /// When [tristate] is false, this value must not be null.
   final bool? value;
 
   /// Called when the value of the checkbox should change.