Basic doc fixes

@greysteil Would you mind taking a peek at this? 
@greysteil Code examples are in separate files eg https://github.com/gocardless/api-docs/blob/basic-doc-fixes/source/docs/07_resources/bill/02_create_a_one_off_bill/z_code/example.php
Are you sure about response? I think that would be more appropriate if this were a request to our servers, but it's just a method call!