Doc for ClientCertificateOptions.Manual refers to class WebRequestHandler only found in .Net 4.8.1

The doc for Manual says:

> The application manually provides the client certificates to the [WebRequestHandler](https://learn.microsoft.com/en-us/dotnet/api/system.net.http.webrequesthandler?view=net-8.0). This value is the default.

Unfortunately, WebRequestHandler is only available in .net 4.8.1 - it is not clear what this option does in .Net core?

Start at the ClientCertificateOptions.Manual API documentation and inspect the linked WebRequestHandler entry across the relevant .NET target frameworks. Determine what Manual means when WebRequestHandler is unavailable, then update the wording so the behavior and supported frameworks are accurate; verify the resulting API documentation.

Scritto dal modello di indicizzazione a partire dal testo della issue.