remove drain-on-drop behavior from vec::DrainFilter and add #[must_use]

library/alloc/src/vec/mod.rs

@@ -2892,6 +2892,12 @@ impl<T, A: Allocator> Vec<T, A> {
     /// If the closure returns false, the element will remain in the vector and will not be yielded
     /// by the iterator.
     ///
+    /// If the returned `DrainFilter` is not exhausted, e.g. because it is dropped without iterating
+    /// or the iteration short-circuits, then the remaining elements will be retained.
+    /// Use [`retain`] with a negated predicate if you do not need the returned iterator.
+    ///
+    /// [`retain`]: Vec::retain
+    ///
     /// Using this method is equivalent to the following code:
     ///
     /// ```