Update mod.rs

Author: invpt

library/alloc/src/vec/mod.rs

@@ -1074,8 +1074,9 @@ impl<T, A: Allocator> Vec<T, A> {
 
     /// Converts the vector into [`Box<[T]>`][owned slice].
     ///
-    /// If the vector has excess capacity, its items will be moved into a
-    /// newly-allocated buffer with exactly the right capacity.
+    /// If the vector has excess capacity, it will shrink down as close as
+    /// possible to the length but the allocator may still inform the vector
+    /// that there is space for a few more elements.
     ///
     /// [owned slice]: Box
     ///