Update CLI help description for --auth option, SingleOrg choice

[tdykstra]

Add "Entra External ID tenants also use SingleOrg." to the existing SingleOrg text in the help text for:

• dotnet new mvc -h
• dotnet new webapp -h
• dotnet new webapi -h
• dotnet new blazorserver -h
• dotnet new blazorwasm -h

This will keep the CLI help in sync with the online documentation; see dotnet/docs#39378.

cc @DavidHoerster