Document Path::parent behavior around relative paths

A relative path with just one component will return `Some("")` as its
parent, which wasn't clear to me from the documentation.

The parent of `""` is `None`, which was missing from the documentation
as well.

library/std/src/path.rs

@@ -2142,7 +2142,10 @@ impl Path {
 
  /// Returns the `Path` without its final component, if there is one.
  ///
- /// Returns [`None`] if the path terminates in a root or prefix.
+ /// This means it returns `Some("")` for relative paths with one component.
+ ///
+ /// Returns [`None`] if the path terminates in a root or prefix, or if it's
+ /// the empty string.
  ///
  /// # Examples
  ///
@@ -2156,6 +2159,14 @@ impl Path {
  /// let grand_parent = parent.parent().unwrap();
  /// assert_eq!(grand_parent, Path::new("/"));
  /// assert_eq!(grand_parent.parent(), None);
+ ///
+ /// let relative_path = Path::new("foo/bar");
+ /// let parent = relative_path.parent();
+ /// assert_eq!(parent, Some(Path::new("foo")));
+ /// let grand_parent = parent.and_then(Path::parent);
+ /// assert_eq!(grand_parent, Some(Path::new("")));
+ /// let great_grand_parent = grand_parent.and_then(Path::parent);
+ /// assert_eq!(great_grand_parent, None);
  /// ```
  #[stable(feature = "rust1", since = "1.0.0")]
  #[doc(alias = "dirname")]