docs: add HTMLs to be produced

Author: kenjis

user_guide_src/source/helpers/form_helper/013.php

@@ -1,3 +1,7 @@
 <?php
 
 echo form_input('username', 'johndoe');
+/*
+ * Would produce:
+ * <input type="text" name="username" value="johndoe" />
+ */

user_guide_src/source/helpers/form_helper/014.php

@@ -8,7 +8,6 @@
     'size'      => '50',
     'style'     => 'width:50%',
 ];
-
 echo form_input($data);
 /*
  * Would produce:

user_guide_src/source/helpers/form_helper/015.php

@@ -2,3 +2,7 @@
 
 $js = 'onClick="some_function ()"';
 echo form_input('username', 'johndoe', $js);
+/*
+ * Would produce:
+ * <input type="text" name="username" value="johndoe" onClick="some_function ()" />
+ */

user_guide_src/source/helpers/form_helper/016.php

@@ -2,3 +2,7 @@
 
 $js = ['onClick' => 'some_function ();'];
 echo form_input('username', 'johndoe', $js);
+/*
+ * Would produce:
+ * <input type="text" name="username" value="johndoe" onClick="some_function ();" />
+ */