Add SslStream missing API documentation

[aik-jahoda]

Summary

SslStream: add missing API documentation.

part of: dotnet/runtime#41446

[opbld33]

Docs Build status updates of commit ab1d912:

⚠️ Validation status: warnings

File	Status	Preview URL	Details
xml/System.Net.Security/SslStream.xml	⚠️Warning	View	Details

xml/System.Net.Security/SslStream.xml

• Line 0, Column 0: [Warning-xref-not-found] Cross reference not found: 'System.IO.Stream.System#IDisposable#Dispose'.

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[aik-jahoda]

I don't know what causes the invalid xref so I created an issue: #4763

[carlossanlop]

The warning is indeed unrelated to your change and you opened an issue for it, so I think we can get this merged since it LGTM.