sealed keyword is not being displayed on signatures

[mairaw]

The signature generated for sealed classes is not showing the sealed keyword.

Compare https://docs.microsoft.com/en-us/dotnet/api/system.servicemodel.instancecontext

with https://msdn.microsoft.com/en-us/library/system.servicemodel.instancecontext

[mairaw]

/cc @dend @joelmartinez

[mairaw]

@joelmartinez this one seems to have never been triaged. PTAL.