Fix std::path::Path::file_name() doc

Author: GuillaumeGomez

src/libstd/path.rs

@@ -1529,8 +1529,7 @@ impl Path {
 
     /// The final component of the path, if it is a normal file.
     ///
-    /// If the path terminates in `.`, `..`, or consists solely of a root of
-    /// prefix, `file_name` will return `None`.
+    /// If the path terminates in `..`, `file_name` will return `None`.
     ///
     /// # Examples
     ///
@@ -1543,6 +1542,14 @@ impl Path {
     ///
     /// assert_eq!(Some(os_str), path.file_name());
     /// ```
+    ///
+    /// # Other examples
+    ///
+    /// ```
+    /// assert_eq!(Some(OsStr::new("foo.txt")), Path::new("foo.txt/.").file_name());
+    /// assert_eq!(Some(OsStr::new("foo.txt")), Path::new("foo.txt/.//").file_name());
+    /// assert_eq!(None, Path::new("foo.txt/..").file_name());
+    /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn file_name(&self) -> Option<&OsStr> {
         self.components().next_back().and_then(|p| {