[ci skip] Improve FV language string instructions
As suggested in #4095
diff --git a/user_guide_src/source/libraries/form_validation.rst b/user_guide_src/source/libraries/form_validation.rst
index be6d1f2..1fde68d 100644
--- a/user_guide_src/source/libraries/form_validation.rst
+++ b/user_guide_src/source/libraries/form_validation.rst
@@ -547,7 +547,10 @@
file: **system/language/english/form_validation_lang.php**
To set your own global custom message for a rule, you can either
-edit that file, or use the following method::
+extend/override the language file by creating your own in
+**application/language/english/form_validation_lang.php** (read more
+about this in the :doc:`Language Class <language>` documentation),
+or use the following method::
$this->form_validation->set_message('rule', 'Error Message');