`Expression<TDelegate>` class is not `sealed`

Looking into [`System.Linq.Expressions.Expression`](https://github.com/dotnet/runtime/blob/8ace7540515483a9bd39a735323aa5a7632dc600/src/libraries/System.Linq.Expressions/src/System/Linq/Expressions/LambdaExpression.cs#L384) declaration, not `sealed` modifier is found.

But `Expression Class` stated opposite:

- https://learn.microsoft.com/en-us/dotnet/api/system.linq.expressions.expression-1?view=net-6.0
- https://github.com/dotnet/dotnet-api-docs/blob/main/xml/System.Linq.Expressions/Expression%601.xml