Update docs

docs/nested-loaders.rst

@@ -15,7 +15,7 @@ Example::
         <a class="email" href="mailto:[email protected]">Email Us</a>
     </footer>
 
-Without nested loaders, you need to specify the full xpath (or css) for each value
+Without nested loaders, you need to specify the full Xpath (or CSS) for each value
 that you wish to extract.
 
 Example::
@@ -40,6 +40,8 @@ Example::
     # no need to call footer_loader.load_item()
     loader.load_item()
 
-You can nest loaders arbitrarily and they work with either xpath or css selectors.
+You can nest loaders arbitrarily and they work with either xpath or CSS selectors.
+If the specified XPath (or CSS) selector does not exist, it will return None.
+
 As a general guideline, use nested loaders when they make your code simpler but do
 not go overboard with nesting or your parser can become difficult to read.