QL etudes: Add further explanation + link

Author: Shati Patel

docs/language/learn-ql/beginner/heir.rst

@@ -160,4 +160,5 @@ What next?
 ----------
 
 -  Learn more about recursion in the `QL language handbook <https://help.semmle.com/QL/ql-handbook/index.html>`__.
+-  Put your QL skills to the test and solve the :doc:`River crossing puzzle <../ql-etudes/river-crossing>`.
 -  Start using QL to analyze projects. See :doc:`Learning QL <../../index>` for a summary of the available languages and resources.

docs/language/learn-ql/ql-etudes/river-crossing.rst

@@ -227,8 +227,11 @@ that returns the resulting path.
    .. literalinclude:: river-crossing.ql
       :lines: 115-117
 
-For now, the path defined in the above predicate ``reachesVia`` just lists the order of cargo items to ferry.
-You could tweak the predicates and the select clause to make the solution clearer. Here are some suggestions:
+The `don't-care expression <https://help.semmle.com/QL/ql-handbook/expressions.html#don-t-care-expressions>`__ (``_``),
+as the second argument to the ``reachesVia`` predicate, represents any value of ``visitedStates``.
+
+For now, the path defined in ``reachesVia`` just lists the order of cargo items to ferry.
+You could tweak the predicate and the select clause to make the solution clearer. Here are some suggestions:
 
   - Display more information, such as the direction in which the cargo is ferried, for example
     ``"Goat to the left shore"``.