add comment explaining usage of CancellationToken.None

src/EditorFeatures/Core/Extensibility/Navigation/NavigableItemFactory.DeclaredSymbolNavigableItem.cs

@@ -28,6 +28,8 @@ public DeclaredSymbolNavigableItem(Document document, DeclaredSymbolInfo declare
                 Document = document;
                 _declaredSymbolInfo = declaredSymbolInfo;
 
+                // Cancellation isn't supported when computing the various properties that depend on the symbol, hence
+                // CancellationToken.None.
                 _lazySymbol = new Lazy<ISymbol>(() => declaredSymbolInfo.GetSymbolAsync(document, CancellationToken.None).ConfigureAwait(false).GetAwaiter().GetResult());
                 _lazyDisplayName = new Lazy<string>(() =>
                 {