Extend TFO docs

[antonfirsov]

Fixes #9910.

There is no way to add remarks for enum values, so I'm adding a long text to the summary. This seems to be a common practice in SocketOptionName docs.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/ncl

[learn-build-service-prod]

Learn Build status updates of commit a3885e9:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Net.Sockets/SocketOptionName.xml	✅Succeeded	View

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