rust: alloc: add missing comment on try_resize

An update missing from Rust 1.57.0. The comment was added in upstream
commit e8e7f6e05cf6 ("Add truncate note to Vec::resize") to `resize`,
but not to our fallible version `try_resize`.

Fixes: 7aaec26 ("rust: alloc: upgrade to 1.57.0")
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

rust/alloc/vec/mod.rs

@@ -2582,6 +2582,7 @@ impl<T: Clone, A: Allocator> Vec<T, A> {
     /// in order to be able to clone the passed value.
     /// If you need more flexibility (or want to rely on [`Default`] instead of
     /// [`Clone`]), use [`Vec::resize_with`].
+    /// If you only need to resize to a smaller size, use [`Vec::truncate`].
     ///
     /// # Examples
     ///