docs: add description how to get boolean attributes

Author: kenjis

user_guide_src/source/helpers/form_helper.rst

@@ -175,6 +175,10 @@ The following functions are available:
 
     .. literalinclude:: form_helper/014.php
 
+    If you want boolean attributes, pass the boolean value (``true``/``false``). In this case the boolean value does not matter:
+
+    .. literalinclude:: form_helper/035.php
+
     If you would like your form to contain some additional data, like
     JavaScript, you can pass it as a string in the third parameter:
 

user_guide_src/source/helpers/form_helper/035.php

@@ -0,0 +1,13 @@
+<?php
+
+$data = [
+    'name'     => 'username',
+    'id'       => 'username',
+    'value'    => '',
+    'required' => true,
+];
+echo form_input($data);
+/*
+ * Would produce:
+ * <input type="text" name="username" value="" id="username" required  />
+ */