Documentation missing

The overloaded method `public int GetHashCode(StringComparison comparisonType)` is not documented. Its is only mentioned it here and there is no guidance when it should be used. The source file says:

> Gets a hash code for this string and this comparison. If strings A and B and comparison C are such that string.Equals(A, B, C), then they will return the same hash code with this comparison C.

See: https://github.com/dotnet/coreclr/blob/master/src/System.Private.CoreLib/shared/System/String.Comparison.cs.

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#### Document Details

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* ID: 46e1d9ef-f477-c47a-da4e-88feb95f6dc9
* Version Independent ID: acedf195-f641-b347-39a9-7b3a43d32e00
* Content: [String.GetHashCode Method (System)](https://docs.microsoft.com/en-us/dotnet/api/system.string.gethashcode?view=netcore-2.1#System_String_GetHashCode_System_StringComparison_)
* Content Source: [xml/System/String.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System/String.xml)
* Product: **dotnet-api**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**

Abre xml/System/String.xml y localiza la entrada String.GetHashCode(StringComparison); después, compárala con el código fuente correspondiente en System.Private.CoreLib/shared/System/String.Comparison.cs. Actualiza la documentación de la API con indicaciones sobre cuándo debe usarse esta sobrecarga y verifica que la página de referencia renderizada de String.GetHashCode contenga la nueva explicación.

Escrito por el modelo de indexación a partir del texto del issue.