Rollup merge of rust-lang#135415 - Harshit933:hard-link-error, r=ChrisDenton

Add `File already exists` error doc to `hard_link` function

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

I tested it with the current rust master version, this error was returned when there is already a link for the file is present.
This was the error returned:
```
[harshit:../Desktop/rust_compiler_testing/hard_link (master|…5)] cargo +stage1 run
   Compiling hard_link v0.1.0 (/home/harshit/Desktop/rust_compiler_testing/hard_link)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.12s
     Running `target/debug/hard_link`
Err(Os { code: 17, kind: AlreadyExists, message: "File exists" })
```
This is my first PR on rust, any suggestions on which issue I can take next are most welcome 😄

Fixes rust-lang#130117

Author: matthiaskrgr

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
 ///