AssemblyBuilder documentation for .NET Core/.NET 5 is stale

Some of the information on the `AssemblyBuilder` class page: https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Reflection.Emit/AssemblyBuilder.xml is describing the behavior of this API on .NET Framework not on .NET Core/.NET 5.

For example:
* All mentions of "saving the assembly" should be removed, the save functionality is not available in .NET Core
* > To get an AssemblyBuilder object, use the AppDomain.DefineDynamicAssembly method.

The `AppDomain.DefineDynamicAssembly` doesn't exist in .NET Core. The way to create the builder object is different now
* Basically any mention of AppDomain are stale as well
* AssemblyBuilderAccess enum has different values in .NET Core - the document doesn't reflect that change