docs: add link from zip to unzip

The docs for `Iterator::unzip` explain that it is kind of an inverse operation to `Iterator::zip` and guide the reader to the `zip` docs, but the `zip` docs don't let the user know that they can undo the `zip` operation with `unzip`. This change modifies the docs to help the user find `unzip`.

Author: beyarkay

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

@@ -469,6 +469,10 @@ pub trait Iterator {
     /// If the zipped iterator has no more elements to return then each further attempt to advance
     /// it will first try to advance the first iterator at most one time and if it still yielded an item
     /// try to advance the second iterator at most one time.
+    ///    
+    /// To 'undo' the result of zipping up two iterators, see [`unzip`].
+    ///
+    /// [`unzip`]: Iterator::unzip
     ///
     /// # Examples
     ///