Merge pull request rust-lang#1 from Centril/redo-vec-set_len-docs-adjust

Explain safety for `vec.set_len(0)`

src/liballoc/vec.rs

@@ -816,6 +816,9 @@ impl<T> Vec<T> {
     /// let mut vec = vec![vec![1, 0, 0],
     ///                    vec![0, 1, 0],
     ///                    vec![0, 0, 1]];
+    /// // SAFETY:
+    /// // 1. `old_len..0` is empty so no elements need to be initialized.
+    /// // 2. `0 <= capacity` always holds whatever `capacity` is.
     /// unsafe {
     ///     vec.set_len(0);
     /// }