Documentation on XmlSecureResolver is out of date

On .NET Core XmlSecureResolver is effectively a NOP and in .NET Core we have removed many CAS related overloads. The document in XmlSecureResolver though is still the same as it was in .NET Framework.

- see note: https://docs.microsoft.com/en-us/previous-versions/dotnet/framework/code-access-security/code-access-security

example outdated docs:
- https://docs.microsoft.com/en-us/dotnet/api/system.xml.xmlsecureresolver?view=net-5.0
- https://docs.microsoft.com/en-us/dotnet/api/system.xml.xmlsecureresolver.-ctor?view=net-5.0