Update docfx filters
Update docfx filters

We used to filter Foo.Types, which is always just a "container"
class for nested protobuf types - but this appears to then *also*
remove the nested types themselves, which is counterproductive.

Without that filter, we get the class we don't want - but we get the
classes we *do* want, which is a reasonable compromise.

Fixes #982.

iantalarico/google-cloud-dotnet,benwulfe/google-cloud-dotnet,iantalarico/google-cloud-dotnet,chrisdunelm/google-cloud-dotnet,chrisdunelm/google-cloud-dotnet,jskeet/google-cloud-dotnet,googleapis/google-cloud-dotnet,benwulfe/google-cloud-dotnet,jskeet/google-cloud-dotnet,iantalarico/google-cloud-dotnet,evildour/google-cloud-dotnet,jskeet/google-cloud-dotnet,jskeet/gcloud-dotnet,googleapis/google-cloud-dotnet,evildour/google-cloud-dotnet,benwulfe/google-cloud-dotnet,evildour/google-cloud-dotnet,jskeet/google-cloud-dotnet,jskeet/google-cloud-dotnet,chrisdunelm/gcloud-dotnet,chrisdunelm/google-cloud-dotnet,googleapis/google-cloud-dotnet