Re-document `nullable` as a valid option for WarningsAsErrors

[JakeYallop]

Describe the issue or suggestion

Passing nullable to <WarningsAsErrors> enables errors for the set of all nullability warnings.

Documentation was added in https://github.com/dotnet/docs/pull/17094, and seems to have been accidentally removed in https://github.com/dotnet/docs/pull/22797.

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Document Details

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• ID: 26ef35c2-6df4-26f8-32b2-9b50eb76c037
• Version Independent ID: e57ac82b-307e-163b-8172-52644b3a6baa
• Content: C# Compiler Options - errors and warnings - C#
• Content Source: docs/csharp/language-reference/compiler-options/errors-warnings.md
• Service: dotnet-csharp
• Sub-service: lang-reference
• GitHub Login: @BillWagner
• Microsoft Alias: wiwagn

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Associated WorkItem - 255547

[gewarren]

@BillWagner There was some data loss here. Maybe check the other files that were deleted in your PR to see if they had any updates while your PR was in progress, that may then have been lost?