Iterator::cycle() — document empty iterator special case

Author: xkr47

library/core/src/iter/traits/iterator.rs

@@ -3028,7 +3028,8 @@ pub trait Iterator {
     ///
     /// Instead of stopping at [`None`], the iterator will instead start again,
     /// from the beginning. After iterating again, it will start at the
-    /// beginning again. And again. And again. Forever.
+    /// beginning again. And again. And again. Forever. Note that in case the
+    /// original iterator is empty the resulting iterator will also be empty.
     ///
     /// # Examples
     ///