mdoc doesn't recognize when the signature has changed and adds FrameworkAlternate instead

[mairaw]

See PR for more context: https://github.com/dotnet/dotnet-api-docs/pull/39

When upgrading .NET Core 2.1 Preview 1 to Preview 2, a couple of signatures have changed. However, mdoc didn't recognize the change and interpreted that the parameters were for a different framework by adding FrameworkAlternate values.

[joelmartinez]

Just adding a bit of context that I forgot to add back when this first happened ... @mairaw, you were able to find a workaround for this, right? So this is definitely an issue, but not one that is blocking AFAIK