Update the Startup.cs to have a placeholder for external auth

[analogrelay]

Right now the Startup.cs has a line like this:

// To configure external authentication please see http://go.microsoft.com/fwlink/?LinkID=532715

It's not clear that the middleware configuration should go there, and not elsewhere in the file. This isn't helped by the docs not being super clear (dotnet/AspNetCore.Docs#1187) but our templates could improve this as well but indicating that external auth middleware should go there.

// To configure external authentication please see http://go.microsoft.com/fwlink/?LinkID=532715
// Place external authentication middleware configuration here

[rustd]

Let's combine them as a single comment as follows.
// Add external authentication middleware below. To configure them please see http://go.microsoft.com/fwlink/?LinkID=532715

[phenning]

TFS: 219293