docs: Clarify Path::starts_with (and ends_with)

Fixes rust-lang#24882

Author: Ulrik Sverdrup

src/libstd/path.rs

@@ -1449,6 +1449,8 @@ impl Path {
 
     /// Determines whether `base` is a prefix of `self`.
     ///
+    /// Only considers whole path components to match.
+    ///
     /// # Examples
     ///
     /// ```
@@ -1457,6 +1459,8 @@ impl Path {
     /// let path = Path::new("/etc/passwd");
     ///
     /// assert!(path.starts_with("/etc"));
+    ///
+    /// assert!(!path.starts_with("/e"));
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn starts_with<P: AsRef<Path>>(&self, base: P) -> bool {
@@ -1465,6 +1469,8 @@ impl Path {
 
     /// Determines whether `child` is a suffix of `self`.
     ///
+    /// Only considers whole path components to match.
+    ///
     /// # Examples
     ///
     /// ```