[ci skip] Add a note about CI_Form_validation::set_data()
diff --git a/user_guide_src/source/libraries/form_validation.rst b/user_guide_src/source/libraries/form_validation.rst
index fa50c6d..be6d1f2 100644
--- a/user_guide_src/source/libraries/form_validation.rst
+++ b/user_guide_src/source/libraries/form_validation.rst
@@ -689,8 +689,12 @@
$this->form_validation->set_data($data);
-Creating validation rules, running the validation, and retrieving error messages works the
-same whether you are validating ``$_POST`` data or an array.
+Creating validation rules, running the validation, and retrieving error
+messages works the same whether you are validating ``$_POST`` data or
+another array of your choice.
+
+.. important:: You have to call the ``set_data()`` method *before* defining
+ any validation rules.
.. important:: If you want to validate more than one array during a single
execution, then you should call the ``reset_validation()`` method