Clarify the behavior of std::fs::hard_link when the second path already exists

[3mp8r3]

Regarding std::fs::hard_link function (https://doc.rust-lang.org/std/fs/fn.hard_link.html)

The function currently doesn't specify what happens if the second parameter (link / "the destination") already exists.

The note for Unix systems and Linux's man pages suggests it will return an error in this case. However, I don't think one should have to read Linux man pages to determine the behavior of a Rust function. It's also not clear if this is the intended Rust behavior across platforms or if the behavior would be platform-specific.

Assuming it's not platform-specific, I think it would be sufficient to add a bullet point to the # Errors section as a second documented error case.

[ChrisDenton]

Closing as exact duplicate of #130117