DOC: Update TrustedIterator doc

src/iterators/mod.rs

@@ -1600,6 +1600,8 @@ send_sync_read_write!(ElementsBaseMut);
 /// (Trait used internally) An iterator that we trust
 /// to deliver exactly as many items as it said it would.
 ///
+/// # Safety
+///
 /// The iterator must produce exactly the number of elements it reported or
 /// diverge before reaching the end.
 pub(crate) unsafe trait TrustedIterator {}