Update porting guide about undefined variable error

docs/docsite/rst/porting_guides/porting_guide_core_2.19.rst

@@ -633,6 +633,31 @@ Valid options are:
 Multiple options can be combined by separating them with commas.
 
 
+Displaying error when undefined variables in var_files
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In previous versions of ``ansible-core``, undefined variables from var_files were silently ignored and did not trigger errors.
+This is now changed and an error will be displayed when undefined variables in var_files are encountered.
+
+.. code-block:: yaml+jinja
+
+    - hosts: all
+      vars_files:
+        - "{{ inventory_dir }}/vars_files/foo.yml"
+
+.. code-block:: ansible-output
+
+    PLAYBOOK: foo.yml ************************************************************************
+    1 plays in foo.yml
+    [WARNING]: skipping vars_file item due to an undefined variable
+    Origin: /examples/foo.yml:6:7
+
+    4
+    5   vars_files:
+    6     - "{{ inventory_dir }}/vars_files/foo.yml"
+            ^ column 7
+
+
 .. _plugin_api:
 
 Plugin API