fix(docs): filter request value props by parts

Previously, it would just show all parts.
It's still not correct though as this isn't necessarily the parts used
in the request value, but only the ones in the response value.

It's as good as it gets though, that's all the information contained
in the json.

gen/youtube3/README.md

@@ -93,11 +93,16 @@ let mut hub = YouTube::new(hyper::Client::new(), auth);
 
 **TODO** Example calls - there should soon be a generator able to do that with proper inputs
 
-## About error handling
+## Handling Errors
+
+# Some details
 
 ## About Customization/Callbacks
 
-## About Optionals in parts (needed for Json, otherwise I'd happily drop them)
+## About parts
+
+* Optionals needed for Json, otherwise I'd happily drop them
+* explain that examples use all response parts, even though they are shown for request values
 
 [builder-pattern]: http://en.wikipedia.org/wiki/Builder_pattern
 [google-go-api]: https://github.com/google/google-api-go-client