CA2101: Specify marshaling for P-Invoke string arguments

I am trying to expose a C function which takes a UTF-8 string to C# (dotnet 5.0). I am getting a warning which does not make sense to me. Here is a simple way to reproduce it, using fopen(3):

[DllImport("libc.so.6", CharSet = CharSet.Ansi, ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
private static extern IntPtr fopen([MarshalAs(UnmanagedType.LPUTF8Str)] string pathname, string mode);

Visual Studio 2019 is reporting a warning:

From the documentation, it seems I need to set CharSet.Ansi in my case:

• https://docs.microsoft.com/en-us/dotnet/api/system.runtime.interopservices.charset?view=net-5.0

and use UnmanagedType.LPUTF8Str:

• https://docs.microsoft.com/en-us/dotnet/framework/interop/default-marshaling-for-strings#strings-used-in-platform-invoke

What did I misunderstood from the documentation ?