[1.8.x] Fixed #25481 -- Added field.help_text to "Looping over a form's fields" docs.

Backport of ac09d22f79 from master

docs/topics/forms/index.txt

@@ -635,6 +635,9 @@ loop:
         <div class="fieldWrapper">
             {{ field.errors }}
             {{ field.label_tag }} {{ field }}
+            {% if field.help_text %}
+            <p class="help">{{ field.help_text|safe }}</p>
+            {% endif %}
         </div>
     {% endfor %}