Add missing docs for {field} and {param} placeholders in validation m…

…essages. Closes #1503

user_guide_src/source/libraries/validation.rst

@@ -446,6 +446,14 @@ Or as a labeled style::
         ]
     );
 
+If you’d like to include a field’s “human” name, or the optional parameter some rules allow for (such as max_length),
+you can add the ``{field}`` and ``{param}`` tags to your message, respectively::
+
+    'min_length' => '{field} must have at least {param} characters.'
+
+On a field with the human name Username and a rule of min_length[5], an error would display: “Username must have
+at least 5 characters.”
+
 .. note:: If you pass the last parameter the labeled style error messages will be ignored.
 
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