Rollup merge of rust-lang#55874 - denisvasilik:docs, r=alexcrichton

string: Add documentation for `From` impls

Hi this is part of rust-lang#51430. I'm a first time contributor, so I started with a small task adding a bit of documentation for From impls.

src/liballoc/string.rs

@@ -2206,13 +2206,40 @@ impl<'a> From<&'a str> for String {
 #[cfg(not(test))]
 #[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 is owned.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// let s1: String = String::from("hello world");
+    /// let s2: Box<str> = s1.into_boxed_str();
+    /// let s3: String = String::from(s2);
+    ///
+    /// assert_eq!("hello world", s3)
+    /// ```
     fn from(s: Box<str>) -> String {
         s.into_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 = 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()
     }
@@ -2272,6 +2299,20 @@ impl<'a> FromIterator<String> for 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()
     }