Rollup merge of rust-lang#136347 - allevo:patch-1, r=Amanieu

Add a bullet point to `std::fs::copy`

I needed to copy a file but I got the following error:
```
Os { code: 2, kind: NotFound, message: "No such file or directory" }
```
After read the documentation, I though the error was generated by the `from` parameter, forgetting the `to` part. Anyway, I got the error because the parent folder of `to` didn't exist.
Even if the documentation explicitly saying `but is not limited to just these cases`, I would like to add this case because I spent 3 hours around it.

This PR just wants to put a mention about it.

Author: matthiaskrgr

library/std/src/fs.rs

@@ -2531,6 +2531,7 @@ pub fn rename<P: AsRef<Path>, Q: AsRef<Path>>(from: P, to: Q) -> io::Result<()>
 /// * `from` does not exist.
 /// * The current process does not have the permission rights to read
 ///   `from` or write `to`.
+/// * The parent directory of `to` doesn't exist.
 ///
 /// # Examples
 ///