When a type is marked `forInternalUseOnly` no docs should be ported

[ericstj]

See https://github.com/dotnet/dotnet-api-docs/pull/11876/files#diff-b0ae656814a099739c4fe0a46da61348a8a4c558613d7f49cf6607babfa061ec

We shouldn't be porting these internal remarks, however we should keep them in source since they are valuable for developers working on RegEx.