Doc clarifications for semantics properties (#24544)

* update semantics properties doc

packages/flutter/lib/src/semantics/semantics.dart

@@ -3171,6 +3171,11 @@ class SemanticsConfiguration {
   }
 
   /// Whether the owning [RenderObject] is selected (true) or not (false).
+  ///
+  /// This is different from having accessibility focus. The element that is
+  /// accessibility focused may or may not be selected; e.g. a [ListTile] can have
+  /// accessibility focus but have its [ListTile.selected] property set to false,
+  /// in which case it will not be flagged as selected.
   bool get isSelected => _hasFlag(SemanticsFlag.isSelected);
   set isSelected(bool value) {
     _setFlag(SemanticsFlag.isSelected, value);
@@ -3187,6 +3192,10 @@ class SemanticsConfiguration {
   ///
   /// The getter will return null if the owning [RenderObject] doesn't support
   /// the concept of being enabled/disabled.
+  ///
+  /// This property does not control whether semantics are enabled. If you wish to
+  /// disable semantics for a particular widget, you should use an [ExcludeSemantics]
+  /// widget.
   bool get isEnabled => _hasFlag(SemanticsFlag.hasEnabledState) ? _hasFlag(SemanticsFlag.isEnabled) : null;
   set isEnabled(bool value) {
     _setFlag(SemanticsFlag.hasEnabledState, true);