added a note about "Save Selection"

extension/ui/pages/help.html

@@ -57,6 +57,8 @@
 						in the context menu</li>
 					<li>With auto-save active, pages are automatically saved every time after being loaded (or before
 						being unloaded if not). </li>
+					<li>Select "Save Selection" from the context menu without selecting any content first to display a
+						selector that will help you choose content by hovering over it with the mouse.</li>
 					<li>Right-click on the SingleFile button and select "Options" to open the options page.</li>
 					<li>Enable the option "Misc. &gt; save to Google Drive" to upload pages to Google Drive</li>
 					<li>You can use the customizable shortcut Ctrl+Shift+Y to save the current tab or the selected tabs.