Add inline docs

Author: Regenhardt

src/JSInterop/Microsoft.JSInterop/src/Infrastructure/TaskGenericsUtil.cs

@@ -89,7 +89,7 @@ public void SetResult(object tcs, object? result)
             // If necessary, attempt a cast
             var typedResult = result is T resultT
                 ? resultT
-                : result == null && default(T) == null
+                : result == null && default(T) == null // ChangeType can't convert null to value types
                     ? default(T)
                     : (T)Convert.ChangeType(result, typeof(T), CultureInfo.InvariantCulture)!;