Add note to binary-tree exercise about duplicate values (#2790)

Just a small note to clarify a minor point of confusion with the
exercise.

Author: randomPoison

src/smart-pointers/exercise.md

@@ -7,7 +7,8 @@ minutes: 30
 A binary tree is a tree-type data structure where every node has two children
 (left and right). We will create a tree where each node stores a value. For a
 given node N, all nodes in a N's left subtree contain smaller values, and all
-nodes in N's right subtree will contain larger values.
+nodes in N's right subtree will contain larger values. A given value should only
+be stored in the tree once, i.e. no duplicate nodes.
 
 Implement the following types, so that the given tests pass.