Auto merge of #74652 - poliorcetics:clarify-vec-drain-doc, r=jyn514

Improve the documentation for Vec::drain

Fixes #73844.

@rusbot modify labels: A-collections, C-enhancement, T-doc, T-libs

Author: bors

src/liballoc/vec.rs

@@ -1274,11 +1274,10 @@ impl<T> Vec<T> {
     /// Creates a draining iterator that removes the specified range in the vector
     /// and yields the removed items.
     ///
-    /// Note 1: The element range is removed even if the iterator is only
-    /// partially consumed or not consumed at all.
-    ///
-    /// Note 2: It is unspecified how many elements are removed from the vector
-    /// if the `Drain` value is leaked.
+    /// When the iterator **is** dropped, all elements in the range are removed
+    /// from the vector, even if the iterator was not fully consumed. If the
+    /// iterator **is not** dropped (with [`mem::forget`] for example), it is
+    /// unspecified how many elements are removed.
     ///
     /// # Panics
     ///