Auto merge of #30670 - emoon:fs-copy-comment, r=steveklabnik

When looking in the documentation I often scan the examples the first thing I do. In these 3 cases it's not obvious which direction the operation happens by adding this comment it makes it more obvious.

r? @steveklabnik

src/libstd/fs.rs

@@ -842,7 +842,7 @@ pub fn symlink_metadata<P: AsRef<Path>>(path: P) -> io::Result<Metadata> {
 /// use std::fs;
 ///
 /// # fn foo() -> std::io::Result<()> {
-/// try!(fs::rename("a.txt", "b.txt"));
+/// try!(fs::rename("a.txt", "b.txt")); // Rename a.txt to b.txt
 /// # Ok(())
 /// # }
 /// ```
@@ -877,7 +877,7 @@ pub fn rename<P: AsRef<Path>, Q: AsRef<Path>>(from: P, to: Q) -> io::Result<()>
 /// use std::fs;
 ///
 /// # fn foo() -> std::io::Result<()> {
-/// try!(fs::copy("foo.txt", "bar.txt"));
+/// try!(fs::copy("foo.txt", "bar.txt"));  // Copy foo.txt to bar.txt
 /// # Ok(()) }
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -896,7 +896,7 @@ pub fn copy<P: AsRef<Path>, Q: AsRef<Path>>(from: P, to: Q) -> io::Result<u64> {
 /// use std::fs;
 ///
 /// # fn foo() -> std::io::Result<()> {
-/// try!(fs::hard_link("a.txt", "b.txt"));
+/// try!(fs::hard_link("a.txt", "b.txt")); // Hard link a.txt to b.txt
 /// # Ok(())
 /// # }
 /// ```