How to deal with cross assembly crefs

I backported docs to System.IO as an experiment. It made FileSystemName (which is in corelib) have `` (which is in a different assembly). This is fine for the docs build because its context is apparently "everything". However the compiler complains with CS1574 and I have to remove the cref.

When making code the source of truth, how can we preserve cref's across assemblies?

@carlossanlop

首先检查 api-docs-sync 在 System.IO 实验期间如何处理三斜杠注释、IntelliSense XML 和 cref 值。复现从 FileSystemName 到 System.IO.FileSystemWatcher 的跨程序集引用，并将文档构建与编译器的 CS1574 诊断进行比较。当 source-of-truth 工作流在不触发该编译器错误的情况下保留该引用时，即视为完成。

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