Small comments on node registration (#399)

Author: AndyZe

docs/tutorial_05_subtrees.md

@@ -44,7 +44,14 @@ It is also the first practical example that uses `Decorators` and `Fallback`.
 </root>
 ```
 
-It may be noticed that we incapsulated a quite complex branch of the tree,
+For readability, our custom nodes are registered with the one-liner:
+
+`CrossDoor::RegisterNodes(factory);`
+
+Default nodes provided by the BT library such as `Fallback` are already registered by
+the BehaviorTreeFactory.
+
+It may be noticed that we encapsulated a quite complex branch of the tree,
 the one to execute when the door is closed, into a separate tree called
 `DoorClosed`.
 

examples/t05_crossdoor.cpp

@@ -61,7 +61,9 @@ int main(int argc, char** argv)
 {
     BT::BehaviorTreeFactory factory;
 
-    // register all the actions into the factory
+    // Register our custom nodes into the factory.
+    // Any default nodes provided by the BT library (such as Fallback) are registered by
+    // default in the BehaviorTreeFactory.
     CrossDoor::RegisterNodes(factory);
 
     // Important: when the object tree goes out of scope, all the TreeNodes are destroyed