Update GitBook docs with warning about core-type scaffold tests failing

docs/book/beyond_basics/controllers_for_core_resources.md

@@ -4,17 +4,25 @@ This book is being actively developed.
 
 ## Controllers for Core Resources
 
+{% panel style="warning", title="Core Only Projects" %}
+If your project will *only* contain controllers for types defined outside your project,
+you must create the project with the `--controller-only` flag.
+{% endpanel %}
+
 {% method %}
 
-It it possible to create Controllers for Core resources, or for resources defined outside your project.
+It is possible to create Controllers for Core resources, or for resources defined outside your project.
 
 {% sample lang="bash" %}
 ```bash
 kubebuilder create controller --group apps --version v1beta2 --kind Deployment --core-type
 ```
 {% endmethod %}
 
-{% panel style="warning", title="Core Only Projects" %}
-If your project will *only* contain controllers for types defined outside your project,
-you must create the project with the `--controller-only` flag.
+{% panel style="warning", title="Scaffold Tests May Not Pass" %}
+When creating controllers for core and existing types, it may be necessary to modify
+the scaffold tests before they pass.
+
+This is because the empty objet may not be valid and have required fields not
+set in the scaffold test.
 {% endpanel %}