clarify why copy.Copy is unsafe

[errordeveloper]

The documentation currently states this:

https://github.com/tonistiigi/fsutil/blob/9e7a6df4857652bc85225f55e2edb8aece3ecd43/copy/copy.go#L65-L66

It's not clear what is the safety concern exactly.

[tonistiigi]

I think this comment is old, before the proper rootPath protection were added. Before that, the concern was that copying paths that contain symlinks could potentially access files outside of the src/dest arguments.