Add docs for From::<&mut str>::from String impl

Author: lopopolo

src/liballoc/string.rs

@@ -2227,6 +2227,9 @@ impl From<&str> for String {
 
 #[stable(feature = "from_mut_str_for_string", since = "1.44.0")]
 impl From<&mut str> for String {
+    /// Converts a `&mut str` into a `String`.
+    ///
+    /// The result is allocated on the heap.
     #[inline]
     fn from(s: &mut str) -> String {
         s.to_owned()