docs: mention permute with replacement

src/lib.rs

@@ -1661,6 +1661,9 @@ pub trait Itertools : Iterator {
     ///
     /// If `k` is greater than the length of the input iterator, the resultant
     /// iterator adaptor will be empty.
+    /// 
+    /// If you are looking for permutations with replacements,
+    /// use `repeat_n(iter, k).multi_cartesian_product()` instead.
     ///
     /// ```
     /// use itertools::Itertools;