Explicitly note that vector contents are on the heap

src/libcollections/vec.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-//! A growable list type, written `Vec<T>` but pronounced 'vector.'
+//! A growable list type with heap-allocated contents, written `Vec<T>` but pronounced 'vector.'
 //!
 //! Vectors have `O(1)` indexing, push (to the end) and pop (from the end).
 //!