Use some StringSyntax attributes

[rainersigwald]

Fill StringSyntax in for some of our APIs.

This is on much shakier ground than my other PR for new language features:

1. Most of the attributes don't actually do the nice syntax highlighting even in VS.
2. This might break folks, like it did in our build. But it's suggestion by default and someone would have needed to opt in--like we do in https://github.com/dotnet/msbuild/blob/030946cbdfc23cae3fe404e49a4292138dd26077/eng/Common.globalconfig#L411.

On balance I think I still want it but if someone disagrees we can close it.