docs: add descriptions

Author: kenjis

user_guide_src/source/libraries/uploaded_files.rst

@@ -83,9 +83,19 @@ this code and save it to your **app/Controllers** directory:
 
 .. literalinclude:: uploaded_files/002.php
 
-.. note:: Since the value of a file upload HTML field doesn't exist, and is stored in the ``$_FILES`` global,
-    only :ref:`rules-for-file-uploads` can be used to validate upload file with :doc:`validation`.
-    The rule ``required`` also can't be used, so use ``uploaded`` instead.
+Since the value of a file upload HTML field doesn't exist, and is stored in the
+``$_FILES`` global, only :ref:`rules-for-file-uploads` can be used to validate
+upload file with :doc:`validation`.
+
+The rule ``required`` cannot be used either, so if the file is required, use
+the rule ``uploaded`` instead.
+
+Note that an empty array (``[]``) is passed as the first argument to
+``$this->validateData()``. It is because the file validation rules get the data
+for the uploaded file directly from the Request object.
+
+If the form has fields other than file upload, pass the field data as the first
+argument.
 
 The Routes
 ==========

user_guide_src/source/libraries/validation.rst

@@ -885,6 +885,8 @@ Available Rules
 .. literalinclude:: validation/038.php
    :lines: 2-
 
+.. _rules-for-general-use:
+
 Rules for General Use
 =====================
 
@@ -1057,6 +1059,8 @@ parameter of all file upload related rules::
         'avatar' => 'uploaded[avatar]|max_size[avatar,1024]',
     ]);
 
+See also :ref:`file-upload-form-tutorial`.
+
 ======================= ========== ============================================= ===================================================
 Rule                    Parameter  Description                                   Example
 ======================= ========== ============================================= ===================================================