Add File already exists error doc to hard_link function

If the link path already exists, the error `AlreadyExists`
is returned. This commit adds this error to the docs.

Author: Harshit933

Diff for: library/std/src/fs.rs

@@ -2529,6 +2529,7 @@ pub fn copy<P: AsRef<Path>, Q: AsRef<Path>>(from: P, to: Q) -> io::Result<u64> {
 /// limited to just these cases:
 ///
 /// * The `original` path is not a file or doesn't exist.
+/// * The 'link' path already exists.
 ///
 /// # Examples
 ///