Issue 61231. Document that Declaration.declaredFragment is null for FieldDeclaration and TopLevelVariableDeclaration.

Bug: #61231
Change-Id: I50b175d8a8bc435947b4336f849960fe2f377846
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/443680
Reviewed-by: Brian Wilkerson <[email protected]>
Reviewed-by: Paul Berry <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov

pkg/analyzer/lib/src/dart/ast/ast.dart

@@ -5620,6 +5620,11 @@ abstract final class Declaration implements AnnotatedNode {
   /// The fragment declared by this declaration.
   ///
   /// Returns `null` if the AST structure hasn't been resolved.
+  ///
+  /// Returns `null` for [FieldDeclaration] and [TopLevelVariableDeclaration]
+  /// because these nodes don't declare any fragments, but individual
+  /// [VariableDeclaration]s inside them do. They are [Declaration]s mostly to
+  /// fit into [ClassDeclaration.members] and [CompilationUnit.declarations].
   @experimental
   Fragment? get declaredFragment;
 }