docs: improve description

Author: kenjis

user_guide_src/source/libraries/validation.rst

@@ -525,7 +525,7 @@ replaced by the **value** of the matched incoming field. An example should clari
    :lines: 2-
 
 .. note:: Since v4.3.5, you must set the validation rules for the placeholder
-    field (``id``).
+    field (the ``id`` field in the sample code above) for security.
 
 In this set of rules, it states that the email address should be unique in the database, except for the row
 that has an id matching the placeholder's value. Assuming that the form POST data had the following: