This fixes a warning when documentation is enabled for a project:
> The file '~/.nuget/packages/fsharp.core/6.0.0/contentFiles/any/netstandard2.1/FSharp.Core.xml' does not specify a 'PublishFolderType' metadata, and a default value could not be calculated. The file will not be copied to the app bundle.
This doesn't change any behavior (as the warning says, the file wasn't copied
to the app bundle before either), but it makes the behavior explicitly
documented and silences the warning.
Fixes https://github.com/xamarin/xamarin-macios/issues/14939.
Fixes https://github.com/xamarin/xamarin-macios/issues/15897.
335f8c9d90