Add note about functions vs trees to node groups page

book/src/api/advanced-scripting/node-groups.md

@@ -22,4 +22,25 @@ The *Instance Grid* node group uses the passed in `instance` argument to create
 
 ![](./instance_grid.png)
 
-This concept can scale to complex interconnected node trees, while keeping everything neatly organized in separate functions.
+This concept can scale to complex interconnected node trees, while keeping everything neatly organized in separate functions.
+
+## Functions vs Node Groups
+
+You do not have to mark a function with `@tree(...)`. If you don't, function calls are treated as normal in Python. No checks are made against the arguments. Any nodes created in the callee will be placed in the caller's tree.
+
+```python
+def instance_grid(instance: Geometry): # Not marked with `@tree(...)`
+    return grid().mesh_to_points().instance_on_points(instance=instance)
+
+@tree("Cube Grid")
+def cube_grid(): # Marked with `@tree(...)`
+    return instance_grid(instance=cube(size=0.2))
+```
+
+The above example would place the *Grid*, *Mesh to Points*, and *Instance on Points* nodes in the main "Cube Grid" tree. It could be rewritten as:
+
+```python
+@tree("Cube Grid")
+def cube_grid():
+    return grid().mesh_to_points().instance_on_points(instance=cube(size=0.2))
+```