Add notions for the tree traversal exercises

subjects/btreeapplyinorder/README.md

@@ -2,7 +2,11 @@
 
 ### Instructions
 
-Write a function that applies a function in order to each element in the tree (see "in order tree walks").
+Write a function that applies a given function `f`, **in order**, to each element in the tree.
+
+### Notions
+
+- [Tree Traversal](https://en.wikipedia.org/wiki/Tree_traversal)
 
 ### Expected function
 

subjects/btreeapplypostorder/README.md

@@ -2,7 +2,11 @@
 
 ### Instructions
 
-Write a function that applies a function using a postorder walk to each element in the tree.
+Write a function that applies a given function `f`, to each element in the tree using a postorder walk.
+
+### Notions
+
+- [Tree Traversal](https://en.wikipedia.org/wiki/Tree_traversal)
 
 ### Expected function
 

subjects/btreeapplypreorder/README.md

@@ -2,7 +2,11 @@
 
 ### Instructions
 
-Write a function that applies a function using a preorder walk to each element in the tree.
+Write a function that applies a given function `f` to each element in the tree using a preorder walk.
+
+### Notions
+
+- [Tree Traversal](https://en.wikipedia.org/wiki/Tree_traversal)
 
 ### Expected function