MAGETWO-91742: "Sort by" not working on Catalog Search results page

- Added hint under configuration.

Author: AlexeyManenok

app/code/Magento/Catalog/etc/adminhtml/system.xml

@@ -83,8 +83,9 @@
                     <backend_model>Magento\Catalog\Model\Indexer\Product\Flat\System\Config\Mode</backend_model>
                     <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
                 </field>
-                <field id="default_sort_by" translate="label" type="select" sortOrder="6" showInDefault="1" showInWebsite="1" showInStore="1" canRestore="1">
+                <field id="default_sort_by" translate="label comment" type="select" sortOrder="6" showInDefault="1" showInWebsite="1" showInStore="1" canRestore="1">
                     <label>Product Listing Sort by</label>
+                    <comment>Applies to category pages</comment>
                     <source_model>Magento\Catalog\Model\Config\Source\ListSort</source_model>
                 </field>
                 <field id="list_allow_all" translate="label comment" type="select" sortOrder="6" showInDefault="1" showInWebsite="1" showInStore="1">