fs::copy and io::copy should See Also each other

[Havvy]

I was pointed at fs::copy on IRC earlier, but couldn't use it with tempfile because that just gives a File. Had fs::copy had a "See also" link to io::copy, I would have found it without complaining that I couldn't use fs::copy. And for similar reasons, io::copy should See Also to fs::copy for those who do want to copy a file based on paths and not readers and writers.

[frewsxcv]

I opened a PR for this: #53376