Mention vec![x; len] syntax in Vec docs

Author: niconii

src/libcollections/vec.rs

@@ -112,6 +112,13 @@ use super::range::RangeArgument;
 /// assert_eq!(vec, [1, 2, 3, 4]);
 /// ```
 ///
+/// It can also initialize each element of a `Vec<T>` with a given value:
+///
+/// ```
+/// let vec = vec![0; 5];
+/// assert_eq!(vec, [0, 0, 0, 0, 0]);
+/// ```
+///
 /// Use a `Vec<T>` as an efficient stack:
 ///
 /// ```