Document behavior of create_dir_all wrt. empty path

The behavior makes sense because `Path::new("one_component").parent() ==
Some(Path::new(""))`, so if one naively wants to create the parent
directory for a file to be written, it simply works.

Closes #105108 by documenting the current behavior.

library/std/src/fs.rs

@@ -2310,6 +2310,9 @@ pub fn create_dir<P: AsRef<Path>>(path: P) -> io::Result<()> {
 /// If this function returns an error, some of the parent components might have
 /// been created already.
 ///
+/// If the empty path is passed to this function, it always succeeds without
+/// creating any directories.
+///
 /// # Platform-specific behavior
 ///
 /// This function currently corresponds to multiple calls to the `mkdir`