Added comments for trait implementations.

src/liballoc/string.rs

@@ -2207,7 +2207,7 @@ fn from(s: &'a str) -> String {
 #[stable(feature = "string_from_box", since = "1.18.0")]
 impl From<Box<str>> for String {
     /// Converts the given boxed `str` slice to a `String`. 
-    /// It is notable that the `str` slice must be owned.
+    /// It is notable that the `str` slice is owned.
     ///
     /// # Examples
     ///
@@ -2227,6 +2227,19 @@ fn from(s: Box<str>) -> String {
 
 #[stable(feature = "box_from_str", since = "1.20.0")]
 impl From<String> for Box<str> {
+    /// Converts the given `String` to a boxed `str` slice that is owned.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// let s1 = String::from("hello world");
+    /// let s2 : Box<str> = Box::from(s1);
+    /// let s3 : String = String::from(s2);
+    ///
+    /// assert_eq!("hello world", s3)
+    /// ```
     fn from(s: String) -> Box<str> {
         s.into_boxed_str()
     }
@@ -2286,6 +2299,20 @@ fn from_iter<I: IntoIterator<Item = String>>(it: I) -> Cow<'a, str> {
 
 #[stable(feature = "from_string_for_vec_u8", since = "1.14.0")]
 impl From<String> for Vec<u8> {
+    /// Converts the given `String` to a vector `Vec` that holds values of type `u8`.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// let s1 = String::from("hello world");
+    /// let v1 = Vec::from(s1);
+    /// 
+    /// for b in v1 {
+    ///     println!("{}", b);
+    /// }
+    /// ```
     fn from(string: String) -> Vec<u8> {
         string.into_bytes()
     }