Docs: Add contextual 'unmanaged' keyword

[avestura]

The unmanaged keyword (constraint) is not on the keywords list.

Example:

class ConcreteClass<T> where T : unmanaged
{

}

---

Document Details

⚠ Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.

• ID: fa5712fd-a965-e2a9-5b1c-0b295d5e8ccd
• Version Independent ID: 4b943400-309d-0437-5513-0533456297f0
• Content: C# Keywords
• Content Source: docs/csharp/language-reference/keywords/index.md
• Product: dotnet-csharp
• GitHub Login: @BillWagner
• Microsoft Alias: wiwagn

[harisriaz17]

Hey, I'd like to fix this issue in the docs.

[harisriaz17]

Ok, I'm beginning work on the PR.

[BillWagner]

I'm going to close this as its covered in the page on generic constraints.

[avestura]

@BillWagner WAIT WHAT! The problem is not that there is no content for it somewhere in the docs. The problem is that "unmanaged" IS a contexual keyword and it IS NOT in the contextual keywords list (read issue's title).
Actually you could simply add the keyword in the list in some couple of days, but you decided to close it after one year...

[BillWagner]

Fair point @0xaryan

I did misread the description going through this. Reopening